Not defending the use of medication, but are we sure he was actually taking any medication at the time he murdered those people? Was he receiving any treatment for his problems? Was he at the very least receiving some sort of counseling?
Hard to say for sure whether he was taking meds that made him do this or whether he was not receiving any care at all.
If he was on meds, how did he get a gun—beg, borrow or steal, or by legal means??