Arjuna didnt even know the truth but still was reluctant to kill karna ... While karna ke life ke 2 mottos the "i am better than arjuna" and "he is my biggest enemy,i am going to kill him"
He also said " 10 thousands krishnas and arjuna together cant defeat me" talk about the ego.
But arjun ke life ka motto tha "jo krishna bole wo sach, aur sab jhooth", "krishn ne bola din to din aur rat to raat", "duniya ka koi bhi dharma krishna se uper nahi"