Its a Threat to Democracy.
The world works on might, there ain't nothing the UN can do to America to stop it doing what it wants to do. International law is meaningless, only works on little tinpot countries, putting sanctions on the US would blow chunks out of the world economy, not to mention it could Veto them anyway. Although, the 2nd Gulf War was legal, because of 1441, the US had the right to inflict serious consequences upon Saddamn.
Every act has a price, but we have to do the greater good. By trying to save everyone, your going to save no-one. Though thousands of people died possibly died in the 2nd Gulf War, Saddamn was killing them by the hundred thousand a year, so it saves more people. Many US and UK troops gave their lives so Iraqi could enjoy life without Saddamn, things might not be perfect right now, but it'll improve, under Saddamn, it'd only get worse.
Really, your being pendantic, equating the US system of law, to using nerve gas on civilians is not smart or impressive debating.
Its in essense its being the worlds policeman, you lock up criminals because they commit crimes, so you kick out dictators because everyone deserves freedom.