'If we decided to use a WMD on one of our own cities - say Cincinatti - because Obama didn't like something the newly elected Republican mayor said, then I'm pretty sure the UN would be scrambling to find a way to punish us'
False equivalence. The Kurds were not the only group Saddam used the weapons against, he also attacked the Iranians. He was vastly outnumbered by the Iranians, but Iraqis had superior technology (from their generous western donors) So first, using weapons against enemies. And second the Kurds had a history of accepting Iranian support (particularly when the Shah of Iran was a US client) and even funding from the CIA via the Shah, in an attempt to secure their independence.
The Kurds were in open revolt at the time that Saddam used chemical weapons against him, and later the no-fly zones were specifically to prevent the Iraqi air-force from bombing Kurds (and in the south Shiite Muslims) - it's not at all like Republicans in Cincinatti, and the UN does not have the ability to punish the US (nor China, Russia, the UK or France)
'We want to be the most the powerful, and keep others from being our equal, because it ensures our survival'
Largely because of this, the UN does what it's most powerful members agree to, and you Krellin, seem to know this. At least on an instinctive level.