Well, given the specific evidence that the culprit dressed up and identified himself as the Joker and informed the police that he had booby trapped his apartment, I'd assign a high probability of schizophrenia, but I wouldn't write off preference based reasons for a sociopath to gun people down at a movie theater in general. It could be motivated by a fantasy rather than a delusion of the perpetrator. Winston Moseley, for instance, sexually assaulted and killed Kitty Genovese and two other women because he had violent sexual fantasies. Alyssa Bustamante (who I read about recently while looking up information on young female murderers,) committed premeditated murder by her own testimony because she wanted to know what it felt like. Personally, I sometimes find myself frustrated at the ineptitude of both terrorists and the Department of Homeland Security, and have thoughts along the lines of "that's pathetic, I could show them how it's done," (I consider this to be one of the manifestations of my imp of the perverse,) but I'm not inclined to do it because I don't want to terrorize the population. If I were a sociopath, on the other hand, I might actually be tempted to do it.
Other people's preferences are not necessarily going to be relatable. Even if there's no potential for profit or cause for vengeance, for the right sort of person, murder could be a thrill activity, like skydiving (another example of something people do for pleasure that other people can't imagine why one would ever want to do it.)
A Ph.D student in neuroscience shot at least 50 people at a showing of the new Batman movie. He also appears to have released some kind of gas from a canister. Because of his educational background this person almost certainly knows a lot about molecular biology. How long will it be (if ever) before a typical bio-science Ph.D will have the capacity to kill, say,a million people?
Edit: I'm not claiming that this event should cause a fully informed person to update on anything. Rather I was hoping that readers of this blog with strong life-science backgrounds could provide information that would help me and other interested readers assess the probability of future risks. Since this blog often deals with catastrophic risks and the social harms of irrationality and given that the events I described will likely dominate the U.S. news media for a few days I thought my question worth asking. Given the post's Karma rating (currently -4), however, I will update my beliefs about what constitutes an appropriate discussion post.