What makes you think NATO nations wouldn't do such a thing, if there were a conventional war between them and Russia? War is essentially irrational, and it causes governments and the general population to desire ends that can only be pursued by harmful, immoral means. What makes NATO immune to this?

In principle nothing, but in practice NATO has shown considerable restraint so far and I see no reason why they would suddenly and catastrophically deviate from that. So unless Russia uses nuclear weapons first - which they have threatened now multiple times - I do not see this or any other conflict NATO is involved in escalate to nuclear.

NATO has never publicly committed to a NFU policy because that would weaken their hand on the political stage but in practice they seem to be following that line. It's a smart way to position themselves, and other nuclear capable entities have done the same.

To add some more context, NATO's refusal to establish a NFU policy dates to the Cold War, when NATO military leaders concluded that defending central Europe from Soviet invasion required a credible threat to deploy tactical nuclear weapons. Confronted with that threat, Soviet forces would have spread out to avoid concentrations large enough to be vulnerable, making a conventional defense feasible by preventing the outnumbering Soviet army from massing to take defended positions, which in turn would provide time for the USA to deploy large armies into Europe. Without this credible threat, the Soviet army would have had the option to pick off a NATO country (i.e. Western Germany) by massing armor columns and forcing a surrender before the USA could deploy a comparable army across the Atlantic.

NATO no longer faces this threat, but the policy remains.

Russia also has not committed to a NFU policy. In fact, they have explicitly stated that, if Russia's survival is threatened (such as, you're rolling tanks over their border), then nukes are in play.

Of course, the problem is that Putin has a very expansive idea of "what threatens Russia"...

Putins ideas of what threatens Russia are not rooted in reality.

I absolutely agree. It's just that, if Putin has these unrealistic ideas of what threatens Russia (like an independent Ukraine), and Putin has stated that Russia can use nukes if it is threatened, and Putin is the one who gets to make the call about whether Russia is threatened enough to use nukes... then I really don't like that one man, disconnected from reality, can start a nuclear war.

Yes, but he's so decoupled from reality that if he wants to do that any pretext will serve so I don't think this will make a difference at all. Putin will do what he thinks he has to do regardless of the rest of the world. It's well beyond logic by now. Russia will suffer for decades after Putin is gone on account of what is happening today no matter how much worse he makes it.

Because NATO would overwhelmingly defeat Russia in a conventional war. There is no real upside and there is very real downside for NATO to escalating a conventional war with Russia to a nuclear war.

The US maintains nuclear ambiguity. A first strike wouldn’t happen under current conditions, but one could easily imagine it happening if the US had credible evidence that Russia was preparing a nuclear attack. Nuclear logic and the ladder of escalation are very terrible things.