I can not accept why people would join IS, but I can definitely understand that being (or feeling) oppressed by a supreme Western military power, seeing your religion vilified and your beliefs (the ones from your holy books, and the ones preached by your preachers, that might be considerably different) disregarded can turn especially young people to extremism.
You know the old saying, one person's terrorist is another person's freedom fighter.
Unfortunately, I see enough vegans who are so saddened by the mistreatment of animals that they would accept violent action (e.g. arson or attacks on vivisectionists) on their behalf, regardless how such action would be seen by the majority (even by the vegan majority).