1. The first nuke used - tactical, Atomic Demolition Munition (ADM) or full-up strategic - will unleash the gates of Hell for the world. They know it, we know it.
2. Nukes are filthy weapons; besides the blast, burn. direct radiation to the target, the indirect radiation, debris, and unconsumed radioactive matter from the bomb itself will be carried upward into the stratosphere and downwind and then all over the world. Everyone, everywhere - and their food supplies - will be in danger. Will make Chernobyl look like a picnic. Downwind in that part of the world includes most of the Middle and Near East and likely China too. New cancers, anyone?
3. It gives lie to the claim that they would be "liberating" the Ukraine - since it would be poisoning the ground for decades to come.
Not only that, Putin doesn’t have a red button directly wired up to nuclear silos, he has to give the orders which are carried out by his generals, and when he does, he will be arrested because those same generals don’t have palatial estates with bunkers to hide out in like Vlad, and know the consequences.
Putin is moving up the nukes
https://www.telegraph.co.uk/world-news/2022/10/03/nuclear-weapons-convoy-sparks-fears-putin-could-preparing-test/
No sane person wants a nuke, but don’t get hysterical. Nagasaki and Hiroshima look better now than Detroit and have for a long time.
Overestimating the damage from a tactical nuke is as bad as underestimating the damage.