I saw a clip of Tony Robbins on youtube(I think at a Ted conference), where he said that people want to be significant, and violence is an easy route. Put a gun to somebody's head and you're important.
I suppose
They want power.. So they have this simple religious ideology islam , convince more people, get more people(islammic propagation), more unity, more power , people notice you, you become more significant. muslim take over. They continuously promote islam , the idea. Even 9/11 made more muslims, more enemies for muslims, but more muslim too. Now everybody had heard of islam.