Iraq did have WMD as of March 2003. It was called Saddam Hussien. Just like Hitler, Stalin, Mao, Pol Pot, etc. were WMDs. WMDs potentially can create great carnage, and that is exactly what those men did.
Thing is, we did not overturn Stalin, Mao, Pol Pot, etc. To some extent, they were our allies. Stalin and Mao were, at any rate. The hypothetical "Good Guys".
At one time, Saddam was the good guy, when we viewed Iran as being more of a problem for overthrowing the pro-US Shah and setting up a fanatical theocracy. That's the reason why there are photos of a much younger Rumsfield shaking hands with Saddam when Rumsfield went to Iraq to offer weapons. We overthrow people not because of how good or bad they are, but because they are no longer useful.
Sorry, but such is the way of things. Might makes right. We are right, because we have the military and/or economic strength to enforce our wishes. Those that do not have the military and/or economic strength to enforce their wishes are wrong, because their opinion is merely an opinion.
"We overthrew Saddam because he was a bad person" is not true. We overthrew him because we wanted to, and because we could. Is the world a better place without him in power? Yep. But there are dozens of other countries with dictators that we will never invade.
"Those that do not rise up and take freedom for themselves do not deserve freedom, nor will they keep it." - An old counterinsurgency instructor