You can't just say that Islam isn't telling them to commit those acts simply because it's violent. What about someone who stops eating due to Ramadan, is that their entirely independent choice or are they doing it because it's part of their religion? Ramadan being a peaceful act makes no difference - it is clearly inspired by the religion. Therefore, as there are many, many lines in the Quran telling people to kill atheists and people of other religions, it is reasonable to assume that terrorists are inspired by their religion just as their religion would inspire them to participate in the peaceful parts of Islam. I don't doubt that they have other issues in their life that may have led them towards embracing a more extreme version of the religion, but to suggest that their actions have nothing at all to do with Islam is ludicrous. The justification is right there in their holy book.