Mental disease is more common than a lot of people realise, someone doesn't have to be a stark raving loony shouting about purple skies and how the babies will eat the wizard to have a mental illness, it can be something simple like a small obsession or a compulsive need to argue and to always be right. Many people suffer from more serious mental illnesses but go throughout life never being diagnosed and with no one ever knowing, they appear perfectly normal and sometimes these people will just snap for no apparent reason.
Sadly gun control focuses too much on banning small inanimate machines and taking them away from perfectly normal law abiding citizens ('cos seriously, if you are liable to do something criminal anyway, do you really give a damn what the law says about owning guns) instead of focusing on the people who are allowed to buy them.

