They’ll alienate the Sunnis. The Sunnis will turn against them. They’ll be destroyed -with our support. This happened to Al Qaeda in Iraq in 2009. It’ll happen to ISIS too, sooner rather than later.
Saddam was a Sunni, who ruled over the Shiite majority of Iraq though terror. During Saddam's reign the Sunnis were in control.
The main target of ISIS terror is the Shiite population of Iraq. They aim to use the kind of terror that Saddam used.