Here's one article I had found about how ISIS recruits non-Muslims (so they aren't all Sunni Muslims). There was a better article I had come across that talked about how some people went to 'work' for ISIS not because of ideology but simply because they paid wages like a job, and there was no work for them otherwise, but they started abandoning ISIS because they were never paid as promised. I can't find that article now, but this one makes my point as well.