Karna was the one of the greatest warrior of Mahabharata.
In an incident which took place during the war between Karna and Arjuna, Ariana's chariot was hit by Karna's arrows and was displaced a few meters backwards from the impact of the arrows. At that time Lord Krishna, who was driving Ariana's chariot, praised Karna for his feat. Arjuna, whose arrows had displaced Karna's chariot by several hundred meters, was shocked at Krishna's praise for Karna and questioned him. Krishna said "Arjuna, I myself who own the whole weight of the entire Universe am sitting on your chariot and your chariot is covered by the blessing of Lord Hanuman. Without these two factors, Karna's arrows would have hurled your chariot off the Earth."
It is further mentioned that Karna was capable of defeating Jarasandha in single combat whereas Bheema defeated Jarasandha with the help of Krishna. Karna was the first one to expose the weakness of Jarasandha.
Finally, Parashurama, before cursing him, had mentioned to Karna that he was his (Parashurama's) complete equal.
Also, Karna had promised Kunti that if, he dies or Arjuna dies, she will always have her five children alive. This might be one of the reason why Karna in spite of King Shalya's suggestion to hit Arjuna on his chest, opted to hit Arjuna on his head, because even Karna knew very well that Sri Krishna will defend Arjuna hence the arrow directed toward the head of the Arjuna could be diverted. Indirectly Karna might be helping Arjuna to escape his arrows. Hence he might be just pretending to attack Arjuna because as per Karna's oath that he would not allow anyone to hurt Duryodhana until he is alive but this oath did not contains a clause that Karna cannot forgive Duryodhana's enemies hence there could be chances that Karna pretended to Kill Arjuna but he would have strictly never allowed Arjuna or any other warrior to kill Duryodhana hence along with being loyal to his friend Duryodhana he could also defend his brothers, indirectly.
Indirectly speaking he sacrificed his life for his friend and for his brother so that there will always be five children of Kunti alive apart from him as it was very well known that he was Dhaanveera so there is no doubt that he can even donate his life for his brother Arjuna. While assuring to Kunti, Karna told that either he or Arjuna will remain, this might be a political statement because Karna who was fighting from Duryodhana's camp cannot directly assure Kunti that he will spare Arjuna hence he might have given her an indication that he will not harm Arjuna because he hinted her that her five childrens will remain alive and at that point of time every one knew that Kunti had only five children, also he rejected her offer to adopt him as her first son because he was obliged to his foster parents to whom he owed the debt of rescuing him and raised him as their own child, also he did not wanted to hurt the dignity of Kunti of bearing a child before marriage hence he asked to remain quiet about his relationship with her like how she was quiet till now. This all indicate that Karna himself accepted death as his future.