We're too quick to use "mental illness" as an explanation for violence.

[Torie]

I had a relative, who is very pacific, punch a pedestrian at random while in a psychotic break. If the voices told him to commit mass murder, would he? Almost certainly not. But if on the edge, or in the zone, with a propensity for violence, and a lot of anger, the psychosis plus factor could well be what pushed this perp over the edge.

Now if he had an accomplice, all of that I think is out the window. Psychotics are loners, and that degree of planning, does not comport for a nutter just doing what the voices tell him to do.

JMO of course, but in this case, I think, a rather educated one. I have taken a course on mental illness, and read many articles on it, and a couple of books. FWIW.