People of the epic · Mahabharata questions
Why did Karna fight for Duryodhana?
Because Duryodhana gave him a kingdom and a standing when everyone else called him a charioteer son. Karna refused Krishna offer of the throne and Kunti appeal to join his brothers, saying that he had eaten Duryodhana bread for years and would not desert him on the eve of the fighting.
The debt is created in a single scene. At the weapons tournament in Hastinapur, Karna outshoots Arjuna and asks for a duel. Kripa demands his lineage, since a prince may only fight a prince, and Karna cannot give one. Duryodhana, in front of the court, makes him king of Anga on the spot so that the duel can be equal. It costs Duryodhana nothing and buys him the most dangerous archer of the age for life.
The epic tests that loyalty twice, hard, and Karna holds both times. On the eve of the war Krishna comes to him privately and tells him he is Kunti eldest son, that by law the kingdom is his, that the Pandavas will follow him and Draupadi will be his queen. Karna answers that Radha is his mother and Adhiratha his father, that Duryodhana has fed him and honoured him for thirteen years, and that he will not now change sides to win. Then Kunti comes herself and asks him to be her son. He gives her the one thing he can: a promise that of her sons he will kill only Arjuna, so that she will still have five when the war ends.
There is resentment in it too, and the epic does not hide that. Karna has been refused, mocked and ranked a half-ratha by Bhishma. His hatred of Arjuna is old and personal, and he is a full participant in the humiliation of Draupadi, telling her she is a woman without honour. He is not a saint driven into the wrong army; he is a man who chose the side that took him.
Krishna says of him afterwards that no man was more generous and none more obstinate in friendship, and that this was both his greatness and the thing that killed him.