Lots of valium, probably in the 100 zillion mg dosage. But when this kind of person thinks they have been wronged (which is approximately all the time), they start acting out. The odd thing is that most people don't see that rage--these kind of people can be very charming when they want something. It's only when you cross them (or he/she thinks you did), that the person lashes out, and that's usually only the person's inner circle--family members, close friends. In the shooter's case, his "acting out" is killing people, one at a time.
The contents of the note that was reported today seems to support this theory. He's very angry at the cops, probably centered on Chief Moose.