I think it's the fact that the Qur'an is an ancient book in modern times. We can't turn a blind eye and pretend these religions have perfect rules and messages. For example, the bible states that women are below men and it endorses slavery based on race. These don't resound with today's values, so we turn a blind eye and believe what we want.
These ISIS people are taking the Qur'an word for word. I'm not too familiar with the Qur'an but I believe this is why they justify their doings.