I don't think - I may be wrong - that Tony Blair wants to intervene to prevent massacres, but to stop the progress of an ultra-extreme fighting force which could further threaten the stability of the region.
I never supported the invasion of Iraq, which was it seems to me extremely misguided and mismanaged, but Iraq was only ever "united" under a sadistic tyrant. Not exactly a multi-cultural paradise :-)
It is hard to believe that even had Saddam survived those divisions would have stayed suppressed. A large part of the blame for what is not happening is down to the current regime, which has stoked Sunnis sense of injustice since the fall of Saddam, and, of course, pernicious outside influences (first al-Qaeda post war and now the overspill from Syria to the west)