Good points.
First, Assad should be held to account for the chemical weapons use in Syria.
And may be:
Criminal Inquiries Loom Over al-Assad’s Use of Chemical Arms in Syria (Published 2021)
"If nothing else, the criminal inquiries in France and Germany could vastly complicate the future for Mr. al-Assad, who has emerged largely victorious in the Syrian war, but with a pariah status that has blocked the international aid necessary to rebuild his country.
Getting such aid could become even more difficult if Mr. al-Assad and his upper echelons are defendants in prosecutions for war crimes in European courts, even if they consider such proceedings illegitimate. "
It moves slowly, but you know the old saying: 'the wheels of justice grind slowly' ('but exceedingly fine') (or 'inexorably' is another wording I've heard, perhaps another saying).
Regarding Russia, it signed onto the Geneva Protocol making use of chemical weapons illegal. So, I think it would be a strong basis for a prosecution of some kind, even if at first in early years only in the meaningful court of public opinion (of the world as a whole). People will see and and remember such evil actions, long before any trial. Putin would no longer be welcome in any Western nation except a client state of little matter. He'd be
considered a war criminal, and that's in the end a bigger punishment than prison.