In plain words
What was the reason? Through bad company Karṇa nursed bad feelings and became one of the wicked four. By joining with these, Duryodhana, Duśśāsana and Śakuni, Karṇa turned wicked. So we must not join bad company. When we join bad company, only bad feelings arise in us. When bad feelings arise, we enter into bad deeds. So company is very important. Karṇa was deeply devoted to Dharma. We must look further into Karṇa's nature. Karṇa fell on the battlefield. Kṛṣṇa resolved to teach Arjuna the greatness of Karṇa. Why was this? Arjuna was caught up in Ahaṅkāra. He thought there was no one in this world who had learned the arts of missiles and weapons greater than he. "Kṛṣṇa is my dear friend, my brother-in-law. He is the one who fulfils my wish": so he grew proud. To crush that pride in a sweet way, Kṛṣṇa took Arjuna along with Him. Kṛṣṇa put on the guise of a brahmin. Karṇa had fallen and was in great distress. He went up to him: Karṇa! You are Karṇa the giver. In My house I have planned a ceremony. In this state it is not right to ask you. But I have come seeking Dharma. Even then, fallen as he was, Karṇa told him to ask for whatever he wanted. "I want a little gold," He said. "Brahmin! Go to my house and say to my wife what I tell you. My wife will give you gold," he said. "I Myself have not come on business. I came for the very purpose of asking you directly. If you give, give; but do not tell Me to ask your wife, your mother-in-law, your father-in-law," He said. Karṇa closed his eyes. On the battlefield, where was there gold with him? He had put gold caps on two of his teeth. In former times this was quite common. When a tooth got a hole, they would close the tooth up with gold. Karṇa said, "Brahmin, take these two teeth away with you." "What is this? Would it be possible for Me to take out your teeth! I would have to hurt you to take the teeth. How can that be a gift? It is no gift. I will not touch your teeth," He said. Then he asked for two stones to be brought. When the two stones were brought and given to him, he knocked out the two teeth with the stones and gave them. "How am I to touch gold that is all blood?" He was still testing him. "You are a kṣatriya. How am I to touch what has been in your mouth? This is against My custom," He said. At once he took an arrow and drove it hard into the ground. The Gaṅgā welled up.