They're extremists, if ISIS is following the Muslim religion, you would see the other one billion Muslims as terrorists too, but no, they're not. Extremists are extremists, it's been like this since the dawn of time, they are not mentally in the correct place, out of the 1 billion + that have the religion as a guide for them I wouldn't be surprised at all if this amount of people are terrorists, not because of the religion but because of them as people, and them just mentally. If these people followed any other religion, they would do the same.