If the Sunnis had stepped down gracefully, I might have some trouble with this. But with Sunnis continuing to kill innocent bystanders because they want to return to their old tyrannical ways, who can blame the Shiites for striking back?
Sooner or later they will need a rule of law. But that means both sides need to stop killing people, not just one of them.
What percentage are the Sunnis in Iraq? Maybe 20-30 %? Not good odds for a civil war against the Shiites who comprise about 60% of Iraq's population.
This is just revenge coming to roost. Can't say as I blame them.