Civil war mass murder is already happening there. I think it was an 'accident' waiting to happen really, after more than a decade of dictatorship, sanctions and repression suffered by the iraqi people, there was bound to be violence involved when Saddam was kicked out.
There would have been a backlash between the minority ruling (Saddam and Co.) and the majority repressed eventually, but the way war/occupation was carried out by USA in such a haphazard, disorganized manner with no future plans for restoring peace, order and development in the country, what's happening now is pretty much uncontrollable... no action that anyone can take will improve or stop it altogether.
It's just a matter of time now when/if both factions will get so sick of fighting that they may eventually call for a ceasefire - it doesnt look very likely for the time being.