I liked the movie good enough, but couldn't quite get behind the hype (I saw people taking 8.5-9/10). I think what threw me a bit (and this could be because I'm not a comic book head), is that I always thought Joker was just an insane guy that wanted mayhem. In this though, his killings were directed at vengeance. I guess to me it's a story of a man snapping, and I get that. But when you make that man the Joker my expectation for his behavior is different.
*SPOILERS FOLLOW THIS*
I may have forgotten someone. Saw it a little while ago. But when I look at who he killed, it wasn't the killings of a chaos loving madman.
He killed the 3 dudes stomping him out on the train for no reason. Justifiable. It was self defense.
His mom. She lied to him all his life, and let him be basically tortured as a kid. You shouldn't smother your mom, but I understand why he did it. It wasn't just to create havoc.
His coworker. That dude was an asshole, and lied on him to get him fired. Not something to kill someone over, but shit... people do. He let the midget go because he was a nice guy.
De Niro. Joker was following his passion/dream and De Niro mocked him on TV, then brought him on to publicly embarrass him again. He didn't try to hurt ANYONE else around him. Just the guy that did that.