Yes its the guy who keeps hinting he's going to use them.
That would be disastrous. That said Putin's invasion being defeated does not lead directly to this scenario. There are scenarios where Putin wins and sels nukes too, equally unlikely.
I disagree, he is literally plundering neighbouring countries under his nuclear umbrella. Some Serb aligned forces used this method in their crime ridden Balkan wars too.
I mean ploughing into Ukraine only to have his arse handed to him is pretty suicidal, or incompetent. I do not advocate regime change in Russia as a policy setting, but we should not avoid resisting Russian expansionism because of the remote possibility of a change of leaders, and that somehow comes up with someone worse than Putin.
There are sensible sounding parts to your argument but the logic gaps are huge. Putin is a present risk, a repeat offender. I don't think the West can force Ukraine to fold just because there's imaginary futures where a Russian leader behaves worse, there isn't much lower such a person could go.