Im not an expert but below is how i imagine some of the russian top brass thinks.

Imagine youre a russian general, you used to command a portion of the red army or your predecessors did. Now in your command you only have gopniks whod rather drink a farmer's cellar dry than listen to orders. The world is making a mockery of your nation for its ineptitude.

Clearly things arent going the russian's way. It doesnt feel like a huge leap of logic for a handful of old dudes with nostalgia for the USSR to decide that if the world wont accept russia, then russia wont accept the world and blow it all up.

In their mind they might not see nuclear holocaust as the end, but rather as a way to "preserve" the strongman image russia once held. In 500 years theyd want russia to be remembered as 1 nation vs the world instead of yet another dictatorship that eventually assimilated with the rest of the world.

I hope im wrong but it feels closer to the truth than id like.