I also did some research, thanks to the web, and discovered that he was also instrumental in the vastraharan of Draupadi. When Draupadi questioned the court how she could be used as a 'bet' when Yudhishtra himself had already lost. Karna in his explanation said that since Yudhishtra himself had lost, it was immaterial what happened later, every of his belongings, now was lost to Dhuryodhana. He further added insult with these commands, "O Duhsasana, seize the garments of the Pandavas and the robes of Draupadi and hand them over to Sakuni".
Why did Karna do this ? Why did he, a supposedly noble person, insult a woman in the court? Was it because she had refused to acknowledge his skills during her swayamvara? Was it because his hatred for the Pandavas was such that he didnt mind this path?
People who talk about his childhood and the injustice, should also not forget the injustice he served unto others. He, tho' noble in several deeds, is surely not the epitome of all that is noble. He was haughty as any other character in the great epic. And was filled with just as much pride. In the end, he was killed because he too was part of wrong.
Krishna had no qualms in killing him. I say Krishna killed him, because Arjuna was just an instrument. Through his Gita, he clearly explained this to Arjuna and us. People can question if the lord can take the law into his hands or if the lord can adopt not so straight forward means. Krishna himself says 'no'. Everyone will pay for his or her actions.
In the entire Mahabharata, Krishna gave ample oppurtunities to either side to exercise prudence and to correct their ways. He even approached Karna to change his ways, and the truth about his brothers. Karna, rightfully stood behind his friend. But he never tried to reason with Dhuryodhana, about his wrongs. Why ? Is it because he knew he was usefull to Dhuryodhana only if he spewed hatred against the Pandavas. What Krishna also highlights with these characters is that, it doesnt matter what the arms, protection, skills, knowledge, friends and family you have to support you? What matters, most, is that Krishna is on your side ?