I wonder what would happen if the world allowed ISIS make their way to within a few hundred, maybe a thousand miles of Israel's borders? It would probably mean the end of the middle east as we know it. Utter carnage.
Eventually ISIS want to conquer all Muslims who don't operate under their "true" caliphate. They want to move into Palestine, destroy Hamas and then annihilate Israel. Israel might let them have a go at Hamas in the first instance, but eventually ISIS would target Israel. Israel is a notoriously twitchy, heavily armed nuclear state. The resulting scrap would be incredibly messy.
It will never be allowed to get to this point, the west and the rest of the middle east will have to step in, properly. These guys are complete nutjobs. They want all of the cake and they want to eat it and they cannot be allowed. There is no place for people like this on the planet. This world is small enough without having to harbour utter scumbags like ISIS.