Why was it our responsibility to deal with that dictator? He never attacked us, and there are lots of horrible leaders in the world (North Korea comes to mind). China is also guilty of uncountable human rights violations. Are we going to invade China too? Is it our responsibility to wage war and free all of these people? Not only can we not afford it, but it is not as black and white as that. A lot of people in Iraq didn't have a serious problem with Saddam. Maybe they weren't overly fond of him, but he was preferable to their homes and families being destroyed as collateral damage in a war that didn't need to happen. The quality of life of a LOT of Iraqi people went down tremendously after we invaded.
I think the dictator thing was an excuse.
We were going to war in that region and the outcome was uncertain. We weren't even really sure what it was that we were going to do, but we knew we had to go and show governments that we wouldn't put up with them allowing terrorists to train on their soil to attacks us. So why not begin the war by taking out the guy with the largest standing army in the region so that we don't have to worry about what he will do?
The freeing the people is how we (by and large) "sell" the war to America... it makes us nobler somehow.
The war was necessary, not as an attempt to stabilize the region... that will never be accomplished, but rather to react to an attack on our soil.
We couldn't just sit and do nothing.
That's just my take.