I suspect to many Muslim youth ISIL looks very little different to the way the left in the Spanish civil war looked to young Europeans last century. Many many Europeans went to fight the dictator Franco and died. Fortunately they did not in the main bring their bombs and guns back with them, we certainly did not put them all in prison as we do returnees from Syria and Iraq.
There is no church hierarchy in Islam that we could expect to make a statement on behalf of the faith, its not so easy to denounce your own religion either. I am not surprised that you think that extremism is tolerated by default in Islam, I think it is more a case of it being difficult to tell by what is pronounced publicly. Also we have to overcome the stunning hypocrisy of Bush and Blair who are personally responsible for lying to the world about weapons of mass destruction then proceeding to start a war in which 153,828 Iraqi civilians have died. This is 51 times more people than died in the Twin Towers. No wonder muslims have concerns about their place in our societies.
Sorry but this is like expecting the Pope to denounce the Westborough hate church. Also remember that the ISIL soldiers in Syria have in fact liberated the country from Bashar al-Assad. The fact that this liberation is into the care of a death cult is something that many will not believe. It would be like the West denouncing a christian militia - say the lords resistance army. I do not recall any church leader excommunicating those men for murder, abduction, mutilation, and child-sex slavery.
The point is that Muslims views are open to persuasion just like westerners. At the moment ISIL is doing a great job of trying to tar Muslims the world over with their greasy death cult and the predictable outraged bigoted red faced old white men are screaming from the rooftops that all Muslims are apologists for terror because young Muslims are being sold a better future by the death cult than the same bigoted red faced old white men.
We need to step up to the plate and offer the youth a better vision than "all Muslims are terrorist sympathizers". If we do not ISIL will succeed in driving a wedge between the west and its Muslim population.
The one guaranteed outcome of us allowing ISIL to do this will be genocide, because they believe that the end times are here and they want it to happen on both sides - and personally I will be blaming the red faced old white men for allowing simple psychology to beat the West if it happens.