Well it's a question of framing. If America is in fact world's police man who protect the rights and freedom of all citizens of the world then America routinely bombs its own Arab citizens and destroys millions of lives to protect their rights.
The only use of weapons of mass destruction by the US was in WWII, and nukes weren't consider off limits yet. Since then we helped to get them banned in as many places as possible, and in general find them too indiscriminate.
Just like the world community agreed chemicals were a bad idea after WWI. Countries using the WMDs use them once in one war and found them so reprehensible that everyone gets together afterward and an agrees that these are too much even in war against hated enemies.
Anyone using any WMD after the hard lessons about them have already been learned is so evil that it is trivially easy to get many armed nations, even those with only the loosest ties, to attack them.
Consider: https://en.wikipedia.org/wiki/List_of_weapons_of_mass_destru...
No, just atomic bombs, agent orange, napalms, drones, depleted uranium, and diseases...
> The magnitude of Assads participation in first inspiring chaos in Iraq,
The Iraq war would qualify as the BIGGEST factor in inspiring chaos in Iraq.
> then purging the Syrian people, and an increasingly brutal hold to power, far out weight any minor US involvement.
Assad is trying to hold on to legitimate power where the US is attempt to purge Syria of his government using terrorists relabeled freedom fighters who have destabilized the entire country. This began as a small movement during Arab spring which was mainly due to food price problems which were out of Assad's control due to the global economy. The situation has totally deteriorated due to American intervention, as it always does. If America never supported the so-called freedom fights, significant human life and livelihood would've been spared.
In the end I don't believe any one in the situation is a good guy. They're all scumbags playing realpolitik.
I think you are widely overestimating both the US involvement and its degree of control over the actors it has links to. I'd be surprised if the main sponsors of the various anti-Assad factions (Saudi Arabia and Qatar) had anything like firm control of the factions they are funding.
The CIA created ISIS. The Congresswoman from Hawaii, who has visited Syria, even says as much: