Even though he killed 12 people and 58 were injured, I don't think he should be executed. He obviously has a mental illness as he stated that he was the Joker. He dyed his hair red. He also was a very smart person, so to go on a massacre like that when he's got his whole life ahead of him just doesn't make sense. There's a chance he can still be rehabilitated and just have a life in prison. It probably won't happen though, given that everyone's going to want "justice". You have to ask yourself if it's really going to make people feel better, or just make people feel empty once they realize that it's another life gone in this tragedy.
Hopefully he doesn't get the death penalty and just gets imprisonment.