I don't think there was a rational motive behind this, or we would have heard about it by now. What the scary truth seems to be is, that these individuals are i

I don't think there was a rational motive behind this, or we would have heard about it by now. What the scary truth seems to be is, that these individuals are in a severely distorted state of perception, in which the killing of people becomes justifiable for them. He has most likely a extreme case of depression, psychosis or something else along those lines, of which the causes need to be investigated. But there will never be a reasonable "why", other then a sad story of a human being, who's life went wrong at one turn or another and in the end his mind malfunctioned. To prevent this from happening, I'd argue, given the amount of people, the complex society we live in and the unfortunate circumstances some people experience in their live, we need to accept that disturbed personalities like these sometimes "happen", and I'm not sure how to avoid that. In my opinion, the most effective prevention would probably be to restrict easy access to lethal weapons. When we realize that (sadly) our society can produce minds like this, we have to make sure you can't buy smoke grenades at a pawn shop.