Blue, genocide and crimes of agression certainly demand action- no doubts about that. but what that action is, and how we mete out justice for these actions, are politically relevant. Not punishing Sudan, for example, or the Saudi Arabians or Israel is just as bad as not going after saddam. How do we explain nailing Iraq but not these others?
Hence the need for an effective international body to enforce international law... too bad the US refuses to recognize the International Court of Justice.
If prior invasion of a country was a legitimate basis for a third country invading, we would be in serious trouble. Further, Western interference in the Middle East has only caused or exacerbated problems there: border disputes, including that between Iraq and Kuwait, stem from the area's history of British colonialism. America interference hasn't exactly been helpful, either (see: rise of the Taliban).
As to evidence of WMD, there was a UN nuclear inspection in the country just prior to our attack, only leaving when it because clear we were charging ahead no matter what. Although Saddaam talked tough about keeping the weapons inspectors out at first, he did relent. We would have a much better idea of exactly what was going on in Iraq, weapons producion-wise, had the weapons inspectors been able to complete their job.