Just as a side note, I will remind you all that Karan's birth was also sort of brought about by fate. Kunti merely wanted to test a wish given to her by some sage that would allow her to have a child in a God's name. Kunti was not even married when she tested this boon and she was not intending to have a son at the time, if I'm not wrong.
A lot of Karan's life is related to fate in general. It's often fate that had shaped Karan's life. There are many, many instances where we can see Karan's bad luck. It is not directly Karan's fault that he did not join God's side in the battle, it was instead that the course of events in his life forced him to join Duryodhan and his brothers. Karan was in full knowledge that the Kauravs did not have an entirely just cause. Karan, himself, did not like some of their actions. Yet, despite all this, he never wavered in his morals or get annoyed with what was happening to him, but instead showed immense loyalty to Duryodhan because he had helped him earlier. Let me remind you that Duryodhan was about the only person who had shown friendship to Karan early in Karan's life. Was Karan supposed to ignore this gesture later and give up their friendship so that he can join the side of God?
Haha, I have a lot of questions about this because the story does not satisfy me entirely. That is why I said that the story about Karan is perhaps the most interesting part of Mahabharata as a whole ...