Isis' main strategy is to claim authority over all Muslims in the world. Meaning that they believe that if you are Muslim, then you should follow their orders or else you are betraying God (allah). With this in mind, they have people spread propaganda to Muslims in the area to get supplies and recruits. Anyone (Muslim or non Muslim) who disagrees with them is either bullied into submission or killed. This is why so many refugees are fleeing into European countries like Germany. Islam as a religion isn't the root of the problem. Clandestine groups are run by fanatics, and these fanatics need to be kept under control or removed. The Ku Klux Klan used to use Christian philosophy to justify killing black people. The difference between Isis and the KKK is that nowadays no one takes the KKK seriously, and the KKK is tiny compared to Isis.