A muslim, particularly an arab muslim country, without a dictator slaughtering people continually = chaos.
These people's natural tendency towards horrific theocracy can only be fought by killing and torturing them wholesale.
I'm not endorsing your premise, but why not just leave Saddam Hussein as the oppressor? Why spend $200-500 billion putting our brutal leadership in charge?
I thought Saddam Hussein was perfect for his setting. However he wasn't staying under our thumb quite like he should.
I think the invasion was a misunderstanding - our enemy is religious, not secular.