In the Kurukshetra war of the Mahabharata, Krishna prompted Bhima to falsely say that Ashwatthama was killed, when Drona was found to be superior to all warriors. Krishna prompted so knowing very well that Drona might even kill Arjuna that day.
My question is:
Why did Krishna took the side of a lie? He himself taught Arjuna about the right principles in the same battlefield. Is there any hidden meaning?
It is not Bhima but Yudhisthira. This is how Krishna defended himself in the Mahabharata.
Krishna defended his actions as follows in a voice deep as that of the clouds or the drum: "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." (Mahabharata, Salya Parva, Section 61)
Not only Bhima, Krishna also had insisted Yudhishtira to lie about Ashwatthama before Drona's death, as it was evident that Drona (aware of his son Ashwatthama's capabilities) may not trust Bhima alone and may approach Yudhishthira to seek the truth.
My question "Meaning of Krishna's advice to Yudhisthira before Drona Vadh" is on the same event but different aspect. Its source actually answers your question:
Govinda, knowing that Drona, that foremost of warriors, was capable of sweeping all the Pandavas off the face of the earth, became much distressed. Addressing Yudhishthira he said, 'If Drona fighteth, filled with rage, for even half-a-day, I tell thee truly, thy army will then be annihilated. Save us, then, from Drona. under such circumstances, falsehood is better than truth. By telling an untruth for saving a life, one is not touched by sin. There is no sin in untruth spoken unto women, or in marriages, or for saving king, or for rescuing a Brahmana. 1
1: This verse is omitted in the Bombay text. There can be no doubt, however, about its genuineness.
Somewhere Krishna suggested Arjuna that to establish Dharma, if little Adharma is required then let it be. For example the fight of Bhurishrava vs Satyaki during Jayadratha Parva:
Krishna told Arjuna, “Burishwara is going to kill Satyaki. You must get Burishwara first before he kills Satyaki.” Arjuna said, “How can I do that? Burishwara is fighting with Satyaki. Shooting him now would be against the Kshatriya dharma.” Krishna told him, “Satyaki has come here only to fight for you. How can you let someone kill him now? Save him.” Arjuna said, “No. It is against the law.” Then Burishwara pulled out his sword in order to decapitate Satyaki. Again Krishna said, “Your friend, who put his life at stake for you, will be slaughtered now if you don’t reach out to him. Will you let him die like this? What kind of dharma is this?” All this time, Arjuna’s arrow was aimed at Jayadrada. When Burishwara was about to decapitate Satyaki, Arjuna turned and shot the arrow at Burishwara. Burishwara’s right hand got severed. He looked around and said to Arjuna, “How could you do this? While I was fighting with another man, you shot me from behind and severed my hand? Anyone who has Kshatriya blood in him wouldn’t do such a dastardly act. I know you wouldn’t have done this by yourself. This cowherd with whom you are must have influenced you.” Arjuna’s anger flared up, and he said, “Yesterday, when you, a great warrior, joined many others to surround my son, a 17-year-old boy, and stab him from behind, where was your dharma? What are you talking about now?” Ashamed of the act that he had committed the day before, Burishwara put his head down. He went next to his chariot and sat down in lotus posture.
Abhimanyu was killed by cheating, while Drona was the Kaurava Army commander. If allowed, Drona ultimately would have got victory for Duryodhana, who was epitome of Adharma. This was the reason, Krishna advised Paandavas to lie.
Ofcourse there would be bad fruit for such misdeed, which Yudhishtira had to undergo before entering heaven. But according to Krishna it was worth for establishing the Dharma.
sarva-dharmān parityajya
mām ekaṁ śaraṇaṁ vraja
ahaṁ tvāṁ sarva-pāpebhyo
mokṣayiṣyāmi mā śucaḥ
Abandon all varieties of Dharma and just surrender unto Me. I shall deliver you from all sinful reactions. Do not fear.
