Who said that Isis are following the Quran? The real question is are they really doing that not because of money? Allah's words are obviously clear, and easy to comprehend, but that doesn't mean that every Muslim is following the Quran, word by word. There're some Muslims who acknowledge that what they are doing is haram, yet they still do it because we humans aren't perfect. Isis are certainly paid. They are incapable of following the Quran because they lost their morality. Anyone who tell you that Isis are following is definitely telling you blatant lies.
Secondly, Allah gave us free will, so if he stopped these people from doing these acts, then he is taking away our free will. In tests, free will are required. You can choose to answer, or to even leave the whole paper empty without answering any question, but eventually you'll regret what you have done.