I don't dislike him for his actions in the beginning of the story since he was just a kid.
I was more confused with his reaction on Accolon vs Lancelot duel. He said himself that he was so horrified that he couldn't do anything. He is an adult person, a king, and he's been a king for 10 years at that point and is still unable to act under pressure. And even after the duel was over Arthur wasn't able to reprimand Lancelot properly.
Again, he is not a bad person. He just sucks as a king