Originally posted by: ..RamKiJanaki..
Karna being invincible with his kavach and kundal does not portray him in a great light. It can be compared to Vali's necklace in the Ramayana. It was said that Indra Dev had given his son Vali a necklace which would make him invincible when he wore it. That is why Lord Ram made him take it off and give it to Sugriva before he died. He said that it's unethical to fight with objects that give one an unfair advantage over their opponent. So Karna had to lose his kavach and kundal, or else people would have said he needed them in order to fight Arjuna, when in reality both were pretty evenly matched in terms of skill.Also, Kunti never asked Karna to spare the other Pandavas and sacrifice himself. She asked him to switch sides and join with his brothers, but Karna told her that was impossible. Still, since she had come to ask him something at sunrise time, he could not deny her completely, so he said he'd spare four of her sons but not Arjuna, and in the end of the war, she would have five sons, whether the fifth be Karna or Arjuna. While Kunti committed mistakes as a mother and did act selfishly at times, she was not that evil that she'd tell Karna to go die and spare his brothers. She was a flawed mother, but she was still a mother who mourned the fact that one of her sons would die.
Even after his Kavach and Kundal was removed, Krishna called him a superior ( If not equal) warrior to Arjuna and told Arjuna that Karna cannot be killed while he has his weapons in his hand.
Actually Karna sparing the 4 brothers shows that he somewhat loved his brothers, why did he give a promise which wasnt even asked?, simple answer - he developed a soft corner for his brothers. No elder brother would like to kill his brothers. As for Arjuna - Karna already said that no-one in the three worlds can harm him while Krishna is with him.