"Insane" is a legal psychiatric term referring to the inability to tell right from wrong due to mental defect. Insanity defenses very very rarely work. I believe (I'm sure someone will end up researching this and correcting me if I'm wrong) insanity defenses typically refer more to psychosis than to psychopathy/sociopathy.
"Psychotic" would refer to some type of disturbance in reality. Hallucinating, delusional, etc. I work with quite a few folks with psychosis. They're quite rarely actually "insane." Some, you'd never be able to tell they have hallucinations, or any type of reality disturbance, to talk to them. In fact, my impression at this point is most people who have psychotic symptoms don't actually have largely psychotic thought processes. Though, the most common psychotic symptoms are very low level (hearing small noises, seeing things out of the corners of one's eyes).
Psychopaths/sociopaths are, for a quick and dirty definition, people who for all intents and purposes "don't have souls."
I'd say that in order to do something as heinous as a mass killing, one would have to be pretty damn sick, but not necessarily "insane" as above. Not necessarily psychotic, either. Sociopathic? Probably. Though, what that says about people in the military who're able to do prolific amounts of killing... is a tangent, but something that does come to my mind. In general, it's scary what human beings are not only able to do to one another, but in many cases, do relatively commonly to one another.
