Things are going just too fast there.
That ISIS militia have just lot of success there, OK of course reasons are too in fact that other side is not so organized, not able to organize anything big without outside help.
But there are logic in those ISIL horrible killings and executions. It is sick logic but it works.
From my personal experience, when your first neighbor (or your fellow soldier or similar) says “they coming, and they killing every man, raping women, and torch everything!” fear can be overwhelming and you just forget everything else.
Reasons, logic, courage, high words, everything goes away when horror coming.
It just work.
It my sound wrong, but good way of dealing with that kind of folks is raising other horror against them by using quick and swift justice, without too much talking. Just destroying.