Forever, people have raped and maimed and killed and claimed that what they were doing was OK because it was justified by their Book of Choice. Half of the Old Testament is made up of stories like this, not to mention the countless empires, kingdoms and realms throughout history who've spread themselves mainly or in part through religious custom. Islam itself spread from a small part of northern Arabia to cover all lands from the Atlas to the Himalayas, in not much over 100 years, and this did not happen through smiles and hugs and laughter. But it was all OK because it was what The Lord wanted, and it said so in The Verses.
Beyond the psychopathy, IS specifically want a caliphate, they're looking at the huge explosion of Islam in the 7th and 8th centuries and they think they can do the same. They think this will make the world perfect, and they think it's what will please The Lord. This movement can't be negotiated with, and I think that's the hardest thing for us (in the Western World™) to deal with, so accustomed are we to negotiation, compromise and relativism.
Blah. I think I just wanted to chime in.