Not far south of where I drink my beer a man killed half a dozen people, shot up or stabbed a couple more. It happened in America but Americans who live just the other side of the border are much like Canadians - they just drink weaker beer and wish Canadian dope could be smuggled across the border as easily as in the days before the border tightened up some.
Straight away the reason given for the murderous spree was that the shooter was mentally ill. The guy is crazy as the buggers who flew those planes into the World Trade Centre.
I have another theory I cannot as yet substantiate. The shooter was brought up a little too Religiously. The Bible makes you crazy. It's true. Just about every mass killer there is fits the same profile. Once you believe one word in a Holy Book you never know when you will start believing the voices in your head telling you to, "KILL EVERYONE NOW!"