if you don't trust the prison system, it doesn't mean you need to kill the criminals to make sure they don't commit crime again.

What happened to Saddam isn't justice, it isn't even close to it, but it's probably the cloesest thing that we'll see in this situation.
not that it'll do any good, five years from now we'll be missing him, but going into iraq was still the right thing to do, becuse one day, after Iraq gets in order (under the rulership of some other dictator) and falls again, the Iraqi people (A false state, by the way, we'll have to wait and see which national groups emerge in the next few years) will have a decent soceity, that will be good both for them and for the rest of the world.