The Psychology of Mass Killers: What causes it? How can you prevent it?

In Las Vegas on 1 October 2017, it appears that one man (although it might have been more) killed 59 people and shot and injured another 241 (with almost 300 more injured while fleeing). The incident got a lot of publicity, partly because the man managed to kill more people than most mass killers. However, because the killer was a white American and had a Christian name, he was not immediately labeled a terrorist, even though his death toll considerably exceeded that achieved in many 'terrorist attacks', including those that occur in war zones (such as US drone murders of innocent people attending weddings).