The Sunnis in Iraq really are a bunch of dead-end morons. There was no way they were ever going to win a civil war given that they constitute only 15 to 20% of the population. So all their insurgency did is show that they are not to be trusted not only because they are thugs, but because they are the worst sort of thugs, which is stupid thugs -- the kind that get lots of people killed for no good reason.
Interestingly enough, the Turks used the Arab Sunni faction as government enforcers and tax collectors. They were allowed to brutalize the Shi'ite majority for many centuries.
As is usual in such cases, the Sunni found it useful to raise their children as sociopaths.
That 15% of the Iraqi people who are sociopathic are not easily dealt with unless they are isolated from everyone else. The Shi'ite militia probably have an idea or two about how that might be done, and there are centuries of revenge to be gotten.