Nothing. They were simply in a situation where they could cosplay the role of good cop by taking control of Assad's chemical weapons as a guarantor that he couldn't use them again. If Putin was desperate (as in, he looks like he's about to lose in Ukraine) he would definitely use them. Losing the war and coming home with nothing to show for it amidst tens thousands of his troops are dead or maimed, is not an option for him since he knows it would lead to his own demise.
Comes down to what does desperation look like? I think if NATO really ramped up their involvement, then the chances of Putin using those weapons becomes higher. For the reasons you state (Biden's "regime change" moment was the closest thing to a "this might mean nukes" scenario I've seen so far, but it was pulled back). As it stands, in a long, drawn out, scenario, I don't see it. I think Russia will take control of the East within a few months and then deal with insurrgency thereafter and Putin won't be long in his role regardless of whatever happens beyond that.
If you're comdemning Russian and Putin, and by logic on the Ukrainian side, then what's the debate ?
The idea that you have to take sides, beyond the obvious "Russia is wrong to invade Ukraine and has done terrible things", is part of the problem. Where's this partisanship in every other conflict currently under way? Do you take sides in the Israeli situation? But beyond side-taking, which is easy, the debate has to do with historiography and causality. The reasonable analyses are macrological, again, as you already know, and factor in many issues which you won't find on your nightly newscast.
When America was at war with Iraq, I don't remember thinking "it would be great if the Iraqis slaughtered Americans", despite America clearly being in the wrong. Debates were more about how/why America should get out and how/why they got there in the first place. It was far more nuanced and reasonable. The bloodthirst (not here, by the way) you see expressed around this conflict is insane.
I'll answer my own point (above). I think it has to do with the Twitterization of news. Maybe, if the Iraq War happened today, we'd see the same thing (people rooting for the Iraqis and compiling lists of the dead and battlezone statistics). The discourse has changed, and I would say largely for the worse. As an example. The American soldiers who came home during the Vietnam War were routinely subject to all kinds of abuse (spat on, beaten, and more). I always thought that was sick. Directing blame/anger where it didn't belong (go to Washington, which they eventually did, and do it there). In this war, social media seems to have amplified the worst aspects of discourse surrounding an already shitty situation and made things exponentially worse.
]). NATO expansion played a role, by the way, which I'm glad folks at CFR can recognize, belatedly, even if others can't. Difference between factors for war and justification of war.
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