My feelings of 'The dark Knight' movie characters

I do sympathize with The Joker for a little bit but i think that he should not have kill people mercilessly.In the movie,he killed people that had helped him in robbing the bank.This shows that he is such an ungrateful person because he killed someone who had helped him.However,it was also told that he was being abused.This is probably the reason why he had became what he is now due to the hurt he had last time.The quotes 'involve child abuse' tell us that he was both physical and psychology hurt.Therefore,from the evidence and quotes,we could see that he was not entirely blame for the state he is in now.Therefore,I sympathize with him a bit but i do not agree of him for killing people in the state of insanity.He should not kill people even though how hurt he was.