We have personnel in the middle east. Unrestricted use of chemical weapons in the middle east is a threat to those personnel, and is not in our national interests.
These are all active military and diplomats. If they get gassed, it will be act of war with the whole world sympathetic. Nobody is that stupid, not when roadside bomb works just as well at actually killing.
We have allies and alliances in effect in the middle east. Unrestricted use of chemical weapons poses a risk to those allies and alliances, and is not in our national interests.
I disagree that any of these allies are worth protecting to that degree. Only if you can demonstrate that chemical weapons could uniquely disrupt the flow of oil would you have a point.
The United States is a signatory to the Chemical Weapons Convention arms control treaty.
The US is also a signatory to the Paris Agreement, but I don't see you advocating carbon tax. There are many other treaties and agreements, including UN, you are simply citing this one because it is convenient.
Again, this is Hollywood sequel of Bush' WMDs. Uncertain information, ulterior motives and inevitable budget-busting expenditures. At best, we will find out how good Russian military tech is. At worst, we will lose a hot war with Russia and embolden them to retake Eastern Europe. In no scenario that I could see after regime change could we end up with Syria as anything other than complete Sharia run by bearded clerics.