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It's an outlet for sociopaths, for one thing. No doubt the ratio of sociopaths to normal people in ISIL is higher than in regular society.
People are also raised in extreme conditions and told that the rest of the world is completely wrong. Everybody's been in a state where they've been outraged to hear someone with an alternate belief than they've always held before, of course. But in some countries, half the Muslim population thinks apostasy should be punished by death. In those places, there is naturally going to be a fringe of even more extreme beliefs, and they aren't going to be able to stand hearing people who think differently. ISIL appeals to that sort of person.0