Karna was the only person who was able to defeat Arjuna. Karna was stronger and kinder than Arjuna, but still he got an undeserved death at the hands of Arjuna,on the advice of Lord Krishna. What is the reason behind ending the life of a great person?
Karna & all other warriors who were cheated (by Krishna) actually should thank PAndavas for giving majorly a fair fight in general. Because the premises of the MahAbhArata war itself was on deceit.
Let's discard the fact that, Duryodhana replaced himself with Shakuni during the dice game and defeated Yudhishitira, because here the latter was also responsible for gambling too many things. But in any case, below things were not justified:
- LAkshAgriha: Attempt to kill PAndavas by burning the palace
- Draupadi's insult: She still had a valid question, which was not answered; Yet, she was insulted
- Breaking the agreement post AgyAtavAsa: It was decided that if PAndAvas finish 12 + 1 years then they will be given their kingdom back with all due respect; But they failed to get even 5 villages
Karna was aware of all these actions, if not involved directly. Still he had decided to fight in side of Duryodhana. Which means he was already fighting for wrong reasons.
(not so) Great person
Being philanthropist doesn't make a person good. There were many evil kings in past who were good in donation and Yagna etc.; e.g. RAvana, JarAsandha. Those were much powerful and bigger than PAndavas + Kauravas combined.
Karna was not stronger than Arjuna. If you gauge strongness by uncompromising attitude, then except Bhima there will be hardly anyone who will pass that test (not even Krishna). Everyone has retreated from war at one point or the other.
There were many duels, between Bhima or Arjuna against Karna and many times the latter had to retreat. These duels were on the might of equal weapons.
During the final battle, wheel of Karna's ratha (car) got stuck into the mud. And he was trying to bring it outside. I feel that, Karna was buying time. This was noted in BR Chopra's episode. He was a mahArathi, hence should have taken the other car, but he din't. It's unclear what his saarathy king Shalya was doing all the time! At the best they should have retreated (like before).
Already being involved in deceitful killing of Arjuna's son Abhimanyu, Karna didn't have any say about morals. It's not that, Arjuna shot the last arrow out of the blue. Karna did plead to Arjuna to wait a while until he gets his wheel out of the mud as mentioned here. During that Karna also asked Arjuna to remember the teachings of righteousness (Dharma). However Krishna replied him:
Then Vasudeva, stationed on the car, addressed Karna, saying, "By good luck it is, O son of Radha, that thou rememberest virtue! It is generally seen that they that are mean, when they sink into distress, rail at Providence but never at their own misdeeds. Thyself and Suyodhana and Duhshasana and Shakuni, the son of Subala, had caused Draupadi, clad in a single piece of raiment, to be brought into the midst of the assembly. On that occasion, O Karna, this virtue of thine did not manifest itself. When at the assembly Shakuni, an adept in dice, vanquished Kunti's son Yudhishthira who was unacquainted with it, whither had this virtue of thine gone? When the Kuru king (Duryodhana), acting under thy counsels, treated Bhimasena in that way with the aid of snakes and poisoned food, whither had this virtue of thine then gone? When the period of exile into the woods was over as also the thirteenth year, thou didst not make over to the Pandavas their kingdom. Whither had this virtue of thine then gone? Thou didst set fire to the house of lac at Varanavata for burning to death the sleeping Pandavas. Whither then, O son of Radha, had this virtue of thine gone? Thou laughedest at Draupadi while she stood in the midst of the assembly, scantily dressed because in her season and obedient to Duhshasana's will, whither, then, O Karna, had this virtue of thine gone? When from the apartment reserved for the females innocent Draupadi was dragged, thou didst not interfere. Whither, O son of Radha, had this virtue of thine gone? Thyself addressing the princess Draupadi, that lady whose tread is as dignified as that of the elephant, in these words, viz., 'The Pandavas, O Draupadi, are lost. They have sunk into eternal hell. Do thou choose another husband!' thou lookedest on the scene with delight. Whither then, O Karna, had this virtue of thine gone? Covetous of kingdom and relying on the ruler of the Gandharvas, thou summonedest the Pandavas (to a match of dice). Whither then had this virtue of thine gone? When many mighty car-warriors, encompassing the boy Abhimanyu in battle, slew him, whither had this virtue of thine then gone? If this virtue that thou now invokest was nowhere on those occasions, what is the use then of parching thy palate now, by uttering that word? Thou art now for the practice of virtue, O Suta, but thou shalt not escape with life. Like Nala who was defeated by Pushkara with the aid of dice but who regained his kingdom by prowess, the Pandavas, who are free from cupidity, will recover their kingdom by the prowess of their arms, aided with all their friends. Having slain in battle their powerful foes, they, with the Somakas, will recover their kingdom. The Dhartarashtras will meet with destruction at the hands of those lions among men (viz., the sons of Pandu), that are always protected by virtue!'"
It was simply because of lack of skill.
During the final battle, the only thing Arjuna did was kill Karna's support and he did not run anything significant against him, although he counteracted some of his weapons. For more information see my answer: War between Karna and Arjuna - Mahabharata
Karna on the 17th was very scary, he even captured Yudhishthira that day but let him go because of his promise to Kunti. Because Karna had his hand on Arjuna (in fact Krishna already knew that something like this was going to happen), he was murdered unjustly simply to obtain the throne of Hastinapura.
Although that if, to justify itself on the murder of Karna, Krishna used of argument its behavior in the sabha:
'Sanjaya said, "Vasudeva was stationed on his chariot. I said, 'O Radheya! It is fortunate that you remember dharma. Quite often, when they are immersed in hardships, inferior ones censure destiny, but not their evil deeds. O Karna! When you, Suyodhana, Duhshasana and Shakuni Soubala brought Droupadi to the assembly hall in a single garment, did dharma not show itself to you? When, in the assembly hall, Yudhishthira, who was not skilled at says, was defeated by Shakuni, who was skilled at says, where did dharma go then? O Karna! During his season, Krishna 798 was under Duhshasana's subjugation in the assembly hall and you laughed at her. Where did dharma go then? O Karna! Resorting to the king of Gandhara and coveting the kingdom, you challenged the Pandavas. 799 Where did dharma go then? 'When Vasudeva addressed Radheya in this way, Pandava Dhananjaya remembered all this and was overcome by great rage. Energetic flames of anger seemed to issue out from the pores on his body and it was extraordinary.
The irony is that Karna lamented all his words towards the Pandavas in his conversation with Krishna, obviously he had to regret his words in the sabha:
'"' O Krishna! I regret the harsh words I used against the Pandavas to please Dhritarashtra's are. I am now tormented by that deed.
In fact that was his only wrongdoing, BORI removed his name from the burning of the lacquer house and the Pandavas or Krishna never blame him for the Bhima poisoning. The most he did was as the saying "add more fuel to the fire" but as described in the previous quote, he regrets those words.
While with the murder of Abhimanyu, he only breaks his bow in a fair fight:
Having heard the preceptor's words, Vaikartana Karna used his arrows to swiftly slice down the bow of the light-handed one. Bhoja 115 killed his horses and Goutama 116 the charioteers who guarded his flanks. Once his bow had been severed, the others enveloped him with a shower of arrows. '"At that time, when he was bereft of his chariot, the six maharathas 117 quickly and ruthlessly showered him with arrows, while the child fought single-handed.
But most likely his arrows were stuck in Abhimanyu's armor because it was described as being a porcupine by the arrows stuck, it is more than obvious that because of his armor. The arrows did almost nothing.
Moreover, Arjuna or Krishna never considered that he cut his bow unjustly or blamed him for his murder, because he only cut his bow (although it was important for his murder)