When I say wide spread support, I'm referring to polls that have consistently showed that the majority are in support of Sharia Law being implemented (or more strictly) or for violent acts against civilian targets. There was an Al Jazeera poll about a year or so ago, where they asked whether they wanted a more secular Sharia Law, and only 33% of those did. The rest, preferred a more violent application of Islam and Sharia Law. Or how about in 2010, when another poll showed that half of Arabs supported Bin Laden. Or just look at the elections that have been had in this countries. Instead of electing moderate, secular governments, they keep electing in religious radicals like in Egypt with the Brotherhood, or in Iraq where they keep electing Maliki. Also, to your point, just because their is violence between muslims, doesn't make them any less dangerous. The whole reason ISIS got a foothold in Iraq, was the ****ty why that the Shia was treating the Sunni. This is why ISIS found such inroads into Iraq, because the Sunnis (of which, ISIS is as well) were sick and tired of being oppressed.
Also, it's not like the MacArthur kids is the first western Muslim to resort to violence against their own country you know? You have the shooter at Ft. Hood for one. The London bombings were carried out by English men.