Why did Bhima kill Duryodhana unfairly by hitting him below the waist during their final battle in the Mahabharata war?
Yudhisthira actually deeply regretted the deceitful actions. He said in Mahabharata Salya Parva Section 59,
"It is we that deserve to be pitied in every respect, O Kaurava! We shall have to drag on a miserable existence, bereft of all our dear friends and kinsmen. Alas, how shall I behold the widows, overwhelmed with grief and deprived of their senses by sorrow, of my brothers and sons and grandsons! ..... We, on the other hand, shall be reckoned as creatures of hell, and shall continue to suffer the most poignant grief! The grief-afflicted wives of Dhritarashtra's sons and grandsons, those widows crushed with sorrow, will without doubt, curse us all!"
Krishna defended the various deceitful actions as follows in a voice deep as that of the clouds or the drum in Mahabharata Salya Parva Section 61:
"All of them were great car-warriors and exceedingly quick in the use of weapons! If ye had put forth all your prowess even then ye could never have slain them in battle by fighting fairly! King Duryodhana also could never be slain in a fair encounter! The same is the case with all those mighty car-warriors headed by Bhishma! From desire of doing good to you, I repeatedly applied my powers of illusion and caused them to be slain by diverse means in battle. If I had not adopted such deceitful ways in battle, victory would never have been yours, nor kingdom, nor wealth! These four were very high-souled warriors and regarded as Atirathas in the world. The very Regents of the Earth could not slay them in fair fight. Similarly, the son of Dhritarasthra, though fatigued when armed with mace, could not be slain in a fair fight by Yama himself armed with his bludgeon! Ye should not take it to heart that this foe of yours hath been slain deceitfully. When the number of one's foes becomes great, then destruction should be effected by contrivances and means. The gods themselves, in slaying the Asuras, have trod the same way. That way, therefore, that had been trod by the gods, may be trod by all."
Krishna argues that Pandavas could not have defeated the Kauravas in a fair fight. A defeat of the Pandavas would have meant defeat of dharma. Hence the deceitful means.....
Let’s first consider how unfair the whole battle was for Bhima. Due to Gandhari’s blessing Duryodhana’s body had become invincible, no matter how expertly or forcefully Bhima hit it. What was he supposed to do? Just let himself get beaten to death for no fault of his – all because his opponent had got a blessing for no virtue of his?
A “blessed” batsman
To grasp the unfairness of the situation, consider a rough cricket analogue: during a faceoff between a champion batsman and a champion bowler, suppose the batsman gets a “blessing” to never get out, even if he is caught, trapped leg before wicket, clean bowled, hit wicket or whatever else. What is the bowler supposed to do? No matter how well he bowls, there’s just no way he can win. Fans with even a modest sense of fairness would be up in arms protesting the way the contest had been rigged against the bowler. But suppose no one protested. The poor bowler bowled the best spell of his life, got the batsman out several times and yet got no credit for it. Wouldn’t that be patently unfair?
That’s what happened to Bhima. He hit Duryodhana twice with such awesome force that the blow could have rent a mountain apart, what to speak of breaking a human frame apart. His achievement was like that of a bowler clean-bowling the batsman not just once but scores of times. And yet what did Bhima get for his feat? Nothing – leave alone victory, not even a crack on Duryodhana’s body.
How long could Bhima go on like this especially when Duryodhana was counterattacking and wounding Bhima? Among the many blows that hit Bhima, two of Duryodhana’s blows were so brutal that they would have instantaneously killed a lesser warrior. Though Bhima was badly injured by those devastating blows, he with superhuman fighting spirit maintained a stoic face, showing no weaknesses. His plight was like that of a bowler carted for six sixes in two successive overs. Actually, Bhima’s plight was a million times worse. Why? Because Duryodhana’s blows were wounding not just Bhima’s morale, but also his body. It’s something akin to the batsman’s shots hitting the bowler, thus rendering him less and less capable of bowling – while still being expected to go on bowling till death.
Can we really blame the battered bowler if he took the only way out of the carnage: bowl bodyline and get rid of the batsman, retired hurt? If we were being wounded like that, can we be sure that we too wouldn’t do something similar?
Understandably, Bhima took the only way out of the slaughter by hitting Duryodhana at the only place it hurt: his thighs. Just as bowling bodyline in normal cricket is unfair, so was hitting the thighs unfair in a normal mace-fighting battle. But when it was the only way to bring some fairness back into an unfair battle, would it still be blameworthy?
Honoring the spirit of the rule, adapting its letter
Rather than blaming Bhima for hitting that unfair blow, perhaps we need to give him credit for fighting fairly for so long, despite being sentenced to an eminently unfair contest. Bhima could have claimed justification for finishing the battle quickly with an early unfair blow:
Maitreya Rishi had cursed Duryodhana that he would die due to the breaking of his thighs and Bhima could have claimed to simply be an instrument for fulfilling the sage’s curse.
Bhima could even have claimed that he had vowed to break Duryodhana’s thighs for having obscenely exposed those thighs to publically humiliate Draupadi – and that he had to do whatever it took to fulfill his vow.
That Bhima did not take recourse of any of these reasons at the first possible opportunity demonstrates his respect for for the spirit of fair play. But the battle had been rendered unfair not because of his action, but even before he took any action.
Their battle was not like a normal mace-fighting battle to which the standard rules applied. Duryodhana had brought something extraneous into the battle – his mother’s protection in the form of an invisible invincible armor. That extraneous factor so totally tilted the scales in Duryodhana’s favor that to re-balance the scales Bhima too had to bring something extraneous – Krishna’s protection in the form of an action plan to bypass that armor. Krishna implemented part A of that emergency plan when he persuaded Duryodhana to cover his private parts while going to see his mother. And then he told Bhima to implement part B of that plan by hitting Duryodhana’s vulnerable thighs.
Yes, that kind of blow was unfair in a normal wrestling battle, but what was normal or fair about a wrestling battle in which one player couldn’t win, no matter how well he fought? Understandably, such an abnormal battle couldn’t be played by the normal rules if there was to be any hope for a fair result.
As Krishna later said, there was no other way Bhima could have won – and so he had to take the only way available.
Duryodhana's upper body was made of vajras (diamonds) and hence no one can kill him by striking anywhere on his upper body. This is explained in Vana Parva of Mahabharata when Duryodhana wants to starve himself to death following his shameful defeat at the hands of Chitrasena in front of the Pandavas. The danavas realizing this summon him to patala and explain him his true purpose.
"The Danavas said, 'O Suyodhana, O great king? O perpetuator of the race of Bharata, thou art ever surrounded by heroes and illustrious men. Why hast thou, then, undertaken to do such a rash act as the vow of starvation? The suicide ever sinketh into hell and becometh the subject of calumnious speech. Nor do intelligent persons like thee ever set their hands to acts that are sinful and opposed to their best interests and striking at the very root of their purposes. Restrain this resolve of thine, therefore, O king, which is destructive of morality, profit, and happiness, of fame, prowess, and energy, and which enhanceth the joy of foes O exalted king, know the truth, the celestial origin of thy soul, and the maker of thy body, and then summon thou patience to thy aid. In days of old. O king, we have obtained thee, by ascetic austerities from Maheswara. The upper part of thy body is wholly made of an assemblage of Vajras, and is, therefore, invulnerable to weapons of every description, O sinless one. The lower part of thy body, capable of captivating the female heart by its comeliness was made of flowers by the goddess herself--the wife of Mahadeva. Thy body is thus, O best of kings, the creation of Maheswara himself and his goddess. Therefore, O tiger among kings, thou art of celestial origin, not human.
So while Duryodhana can be defeated in battle he cannot be killed, which is why Bhima had to strike him below the waist following Krishna's advice:
"'Vasudeva said, "The instruction received by them hath been equal. Bhima, however, is possessed of greater might, while the son of Dhritarashtra is possessed of greater skill and hath laboured more. If he were to fight fairly, Bhimasena will never succeed in winning the victory. If, however, he fights unfairly he will be surely able to slay Duryodhana. The Asuras were vanquished by the gods with the aid of deception. We have heard this. Virochana was vanquished by Shakra with the aid of deception. The slayer of Vala deprived Vritra of his energy by an act of deception. Therefore, let Bhimasena put forth his prowess, aided by deception! At the time of the gambling, O Dhananjaya, Bhima vowed to break the thighs of Suyodhana with his mace in battle. Let this crusher of foes, therefore, accomplish that vow of his. Let him with deception, slay the Kuru king who is full of deception. If Bhima, depending upon his might alone, were to fight fairly, king Yudhishthira will have to incur great danger.
You can further read this answer to understand how this act of Bhima is justified.
Bhima killed Duryodhana by striking his mace towards Duryodhana because Duryodhana earlier received a curse already that he Bhima would crush his thighs. The Mahabharata, Book 3: Vana Parva: Aranyaka Parva: Section X.
After the exile of Pandavas, Maitreya visited them and Pandavas told him about the dice game, the disrobing of Draupadi and also of the 13 years of exile.
He tells this here
"Maitreya said, 'Setting out on a pilgrimage to the different shrines, I arrived at Kuru-jangala, and there I unexpectedly saw Yudhishthira the just in the woods of Kamyaka. And, O exalted one, many Munis had come there to behold the high-souled Yudhishthira, dwelling in an ascetic asylum, clad in deer-skin and wearing matted locks. It was there, O king of kings, that I heard of the grave error committed by thy sons and the calamity and terrible danger arisen from dice that had overtaken them. Therefore, it is that I have come to thee, for the good of the Kauravas, since, O exalted one, my affection is great for thee and I am delighted with thee! O king, it is not fit that thy sons should on any account quarrel with one another, thyself and Bhishma living. Thou art, O king, the stake at which bulls are tied (in treading cord), and thou art competent to punish and reward! Why dost thou overlook then this great evil that is about to overtake all? And, O descendant of the Kurus, for those wrongs that have been perpetrated in thy court, which are even like the acts of wretched outcasts, thou art not well-thought amongst the ascetics!'
He further tells how Bhima slayed strong people.
"Vaisampayana continued, 'Then turning to the wrathful prince Duryodhana, the illustrious Rishi Maitreya addressed him in these soft words, 'O mighty-armed Duryodhana, O best of all eloquent men, O illustrious one, give heed unto the words I utter for my good! O king, seek not to quarrel with the Pandavas! And, O bull among men, compass thou thy own good as also of the Pandavas, of the Kurus and of the world! All those tigers among men are heroes of high prowess in war, gifted with the strength of ten thousand elephants, with bodies hard as the thunderbolt, holding fast by their promises, and proud of their manliness! they have slain the enemies of the celestials--those Rakshasas capable of assuming any form at will, such as were headed by Hidimva and Kirmira! When those high-souled ones went from hence that Rakshasa of fierce soul obstructed their nocturnal path even like an immoveable hill. And even as a tiger slayeth a little deer, Bhima, that foremost of all endued with strength, and ever delighted in fight, slew that monster. Consider also, O king, how while out on his campaign of conquest, Bhima slew in battle that mighty warrior, Jarasandha, possessing the strength of ten thousand elephants. Related to Vasudeva and having the sons of king Drupada as their brothers-in-law, who that is subject to decrepitude and death would undertake to cope with them in battle? O bull of the Bharata race, let there be peace between thee and Pandavas! Follow thou my counsels and surrender not thyself to anger!
Duryodhana later slapped his thigh due to which Maitreya cursed him
Duryodhana began to slap his thigh resembling the trunk of the elephant, and smilingly began to scratch the ground with his foot. And the wicked wretch spake not a word, but hung down his head.
By this Maitreya became angry and he set his mind upon cursing Duryodhana! And then, with eyes red in anger, Maitreya, touching water cursed Duryodhana as follows:
'O king, thus admonished by Maitreya, Duryodhana began to slap his thigh resembling the trunk of the elephant, and smilingly began to scratch the ground with his foot. And the wicked wretch spake not a word, but hung down his head. And, O monarch, beholding Duryodhana thus offer him a slight by scratching the earth silently, Maitreya became angry. And, as if commissioned by fate, Maitreya, the best of Munis, overwhelmed by wrath, set his mind upon cursing Duryodhana! And then, with eyes red in anger, Maitreya, touching water, caused the evil-minded son of Dhritarashtra, saying, 'Since, slighting me thou declinest to act according to my words, thou shalt speedily reap the fruit of this thy insolence! In the great war which shall spring out of the wrongs perpetrated by thee, the mighty Bhima shall smash that thigh of thine with a stroke of his mace!
Dhritarashtra was later worried as to how to nullify the curse and Rishi Maitreya said, the curse would be true if Duryodhana accepts the peace proposal of the Pandavas but the war happened despite the fact that Sri Krishna himself brought the peace proposal so Bhima killed Duryodhana in that manner.
'When the Muni had spoken so, king Dhritarashtra began to pacify the sage, in order that what he had said might not happen. But Maitreya said, 'O king, if thy son concludeth peace with the Pandavas, this curse of mine, O child, will not take effect, otherwise it must be as I have said!'
Krishna also said the act of Bhima is righteous.
The Pandavas of unsullied prowess are our natural friends. They are the children of our own sire's sister! They had been greatly afflicted by their foes! The accomplishment of one's vow is one's duty. Formerly Bhima had vowed in the midst of the assembly that he would in great battle break with his mace the thighs of Duryodhana. The great Rishi Maitreya also, O scorcher of foes, had formerly cursed Duryodhana, saying, 'Bhima will, with his mace, break thy thighs!' In consequence of all this, I do not see any fault in Bhima!