Targeting religion is not quite fair, to be sure. It's the general war on rationality that is causing so much damage.
Remember the anarchists of the late 19th century? The Bolsheviks in Russia? The long Cold War? That was primarily fueled by atheistic progressivism, and killed more people than all the religious wars in history put together.
The violent extremists disagree with you. Abdelhamid Abaaoud, the organizer of the Paris attack, said his goal was to kill as many non-believers as possible for Allah.
ISIS kills believers of Allah in Syria and other places because it's trying to solidify it's power as a government.
ISIS is attempting to start an Islamic Caliphate, considers anyone standing the way of them do so as an apostate, and kills them as such.