Karna was more powerful, he was a great donor, but these virtues alone doesn't make one a good man. He knowingly took the side of Kauravas to maintain the loyalty of friendship even by knowing that his friend is treading the path of unrighteousness. So by siding with the evil party he had made his destiny clear. Moreover, he learnt the art of war from his guru basing upon a lie that he is a Brahmin. So when in the battle his chariot sunk into earth he asked Arjuna for a fair fight and told him to wait for sometime. But Shri Krishna asked Karna where was his sense of virtue while doing many wrong things in the past. Reminding of his past wrong doings Shri Krishna said as below:
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 Duryodhana 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 Krishna, 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.
Realizing his fault Karna didn't give any answer and bowed down his head in shame. Raged with anger he then tried to fight but in course of events got killed by Arjuna. So for the reasons pointed out by Shri Krishna it was no cheating to kill Karna unfairly because he himself did unfair things first siding with the Kauravas.