I saw an article maybe 6 months ago that went into how crazy people don't kill people. Pissed off people do. Whether there's a rational basis in the anger is another story... I couldn't find it again, but I found a few others. In the interest of not having just links to stupid outrage factories like Fox News that blame everything except what really matters, here are some sane articles I found:
People who commit acts of violence lack impulse control. Note that the impulse is almost always anger.
http://www.scientificamerican.com/article/anger-management-self-control/
Psychology of mass murderers. One important thing is that the rate hasn't changed. People just think the world is going crazy because they're reported all the time, so you hear about them more. Also, "
The mass murderer has festering 'real or imagined grievances, frustrations, disappointments, and outrages done to him by others over a long period of time'" sounds a lot like Elliot.
http://www.drtomoconnor.com/4050/4050lect07.htm
So yeah. He fit the profile of a mass murderer. It's not just about insanity in general, but anger at the world, complete lack of empathy, and lack of impulse control.