Sadam Hussain was leader of Iraq solely because he controled the most powerful violent force there. Consequently, by his own principled, the invasion of Iraq by the Coalition was equally legitimate so long as they were able to supplant his monopoly of force with their own, as holding such a monopoly was all he needed to claim to be Iraq's ruler.

The problem with this is that you let the worst people set the rules. Why should the rest of the world evaluate the U.S.'s invasion of Iraq based on Hussain's view of the world and not, international law, UN statements of principal, the U.S.'s stated foreign policy, etc.?

I think the U.S. should be held to its own standards with respect to the invasion of Iraq in which case the Iraq invasion was justified if Hussain had weapons of mass destruction, which was the case that was made to the UN at the time. He didn't, so it wasn't.