The chance of a nuclear encounter is always non-zero. There could always be someone who instead of shooting up a school or church decides to shoot up the world.

The chance of a nuclear encounter is always non-zero. There could always be someone who instead of shooting up a school or church decides to shoot up the world. Or someone sell a nuke to terrorist. If you want to talk probabilities, well, what is the new probability? The answer is that no one has any clue how to even evaluate the probability. And what if the worst happens: Putin says "I'm going to nuke somebody unless you give me Ukraine"? Or he feels like he's losing and just unilaterally decides to take the world down with him? You can't really do anything about those scenarios, so you pretty much have to just assume he won't do that and act accordingly and try not to push him into a scenario where that looks like a good option.