The key thing to understand is that ISIS is first and foremost a criminal and terrorist organization, not a religious group. They use religion (or rather a perverted interpretation of it) to construct a unifying framework, a story for its members, something that they use to silence their members' potential critical thinking.

Don't think for one second that the higher-ups in ISIS and the ones who planned this recent attack are religious people or drink their own ideology kool-aid. ISIS leaders are criminals and do whatever it takes to advance their "careers".