Both options would be a major escalation. On the moral side of things it would put Russia in the ultimate villain class. Either option could make sure Ukraine gets real, active allies on the ground, in the sky and at sea. If you are Russian and feeling more than a little crazy which of these two would you pick? This is only a thought experiment since once you use either one you can just use both. You’d have crossed such a red line it wouldn’t make much of a difference any more.
Pay-off for the Russians: could be useful to flush out defenders in places like Mariupol. Am guessing Ukrainian soldiers have some way of protecting themselves though, so the actual impact may be limited. Civilians would have next to no protection.
Down-sides: This would further destroy Russia’s credit as belonging to the civilized world. The use of chemical weapons can contaminate areas you yourself wish to occupy, so you could be ‘soiling your own nest’. The west will step up its efforts to aid Ukraine. This is a bit strange, but it’s a fact, humans judge killing methods differently. It’s bad to kill Ukrainian soldiers with artillery shells and bullets, but the use of gas is somehow worse. This article does not seek to explain why the world has agreed on this difference in moral weight. Because of the psychological baggage that comes with chemical weapons or gas more Russians may turn their back on their own nation. Even the Russians may judge this as a step too far.
That’s many down-sides for very little pay-off.
Tactical nuclear weapons
Pay-off: If you drop a small nuclear weapon on the center of Mariupol then those circa 5,000 to 10,000 defenders are definitely destroyed as a military factor. You could destroy the defenses around Kiev and/or Charkov in one fell swoop as well. If you can’t take Odessa you can destroy its harbor so it becomes useless. To avoid civilian casualties you could also use tactical nuclear weapons to destroy logistical infrastructure in the rear. The Ukrainians are never going to simply surrender according to me, but facing the use of nuclear weapons and no response in kind from the west the Ukrainians may be willing to substantial concessions allowing the Russians to claim victory. Down-sides? About the same as with chemical weapons, but you are much more certain of destroying your enemy. The biggest risk is that the west would retaliate with its own nuclear weapons, but since that amounts to ‘assured mutual destruction’ perhaps the Russians could decide this war by using them… They would be only the second country in human history to have used nuclear bombs.