Originally posted by: Sabhayata
the way i understand is that after swayambar karna withdraws because he is surprised that a brahmana can fight so well and infact hurt him as well because arjuna does fire something that faints karna so i guess he was taken aback by this brahmana and didnt want to fight further since his previous experience wasn'tt good with brahman's perhaps didnt want to get cursed again
regarding his kavach karna cant be killed due to his kavach but he can be defeated after all Bhima did defeat karna with his kavach and kundal during rajsuya yajna.So he can be injured or defeated but not killed and arjuna's aim was to kill him hence kavach and kundal had to go