Freewill, there will always be a tiny minority of extremists and mentally ill people who become obsessed with all sorts of things and can usually be found in history from different political persuasions, religions and whatever else you can think of.
I don't know how these sort of people can be stopped. Mentally ill people with obsessions are difficult to keep under control, let alone what goes on in their brains, and extremists groups tend to feed each other into frenzies. It's very sad and the implications on innocent people can be horrendous as we have just seen.
