Saddam killed people to keep himself and his regime in power.
We would be killing the extremists to try to give the normal Muslims a chance to live free lives.
Regardless, we can't win at this point. If we go in with overwhelming force, all of the countries (and some American citizens) that have been sitting on the sidelines will start wailing about the illegal war and will want to bring Bush up for war crimes. If we leave, we will be indirectly responsible for the slaughter that will follow.
We are already responsible for the killings there by invading Iraq. If we leave, nothing will change. The only way Iraq can be peaceful again is through genocide or exile of the sunnis or shittes. If we leave, sunnis will still be killing shittes and now shitte milias have begun killing amerian troops.