In plain words
He knows too that by doing that good he will reach a good state. But his mind does not consent to do good. What is the reason for this? Simply that the antaḥkaraṇa behind his will is not pure! Today man's antaḥkaraṇa is filled entirely with selfishness. The plant called life has been sprouting in the very soil called selfishness. For their own selfish ends and their own gain, the Kauravas kept on making the Pāṇḍavas suffer. They knew this was wrong. Yet they did this wrong knowing it. If one does it without knowing, it is a mistake. If one does it knowingly, it is sin. That is all the difference there is between a mistake and a sin! In the matter of gratitude Karṇa set a great example. He was not only a great embodiment of sacrifice. He was ready at any time to dedicate his life to Duryodhana, his master.
At the display of skill in arms by the Kauravas and Pāṇḍavas, Karṇa too came forward, saying he would take part. ‘Only kṣatriyas and princes are entitled to take part in this. Who are you?’ they asked. ‘I have no kingdom at all. I am a sūta. I am not a kṣatriya,’ he said; at that they told him he was not entitled and should go back. Though he had such strength, though he was a man of great virtue and great self-sacrifice, there are certain rules for gaining worldly results. That moment brought him great shame. Before he could go away with bowed head, Duryodhana stepped into his way. ‘After Arjuna there is no other man in this world as mighty as this. I am one who has taken up enmity against the Pāṇḍavas. With a Karṇa like this on my side I can destroy the Pāṇḍavas some day’: with this evil intent he did Karṇa a favour. What did he do? That very moment, in that great assembly, he made Karṇa ruler of the kingdom of Aṅga. In gratitude for that, Karṇa took a vow to offer his whole life to Duryodhana.