ISIS is composed of "Muslims around the world". Not all of them, obviously, but it's rather worrying how widespread their support is. You would think they would be immediately condemned by anyone sane enough to live outside a padded cell, but apparently not.
I dont think they have any widespread support for their tactics, also I haven't noticed large support for them outside of salafi community.
Another thing to point out is that their propagandists, online, are lying and trying to deceive any muslim or nonmuslim who expresses concern. They say that the yazidis are not going to be harmed, neither are the christians. And then you have those fools believing them. I expect them to grow more unpopular over time, and I think they already are to a large degree