Me too.
I wonder if he was on some sort of anti-psychotic med..so often, after an incident of this type, it turns out to be the case.
Assuming this is the case, I’m not trying to say it was the meds that caused him to start shooting (I’m no doctor), just that it usually turns out there’s a history of psychiatric problems.
If so, he may never be able to coherently articulate WHY he went berserk. His reasoning may make sense to him, but it won’t to anyone else!