Saddam Hussein didn't exactly handle the situation well. If he'd been willing to work with the UN, there would not have been a war. He gave the US a great opportunity to attack, and they took it. Why hasn't the US invaded Iran, Pakistan, or North Korea? It's not because they don't want to, and they've got WMD's (Iran is close), it's because those countries maintain some semblance of diplomacy and make some effort to appease the UN. Iraq didn't work with the UN, and the US took advantage.
Also, the US isn't trying to take over Iraq. They just want an Iraqi leadership that they can get along with. I think trying to force democracy upon the population is misguided, but it's not inherently evil.