He has MAD to fear the simple fact that any nuclear strike would be met by a massive counterstrike which would end up with both sides being destroyed, Mutually assured destructions is the basic understanding that has kept humanity from blowing itself off the face of the planet
I agree with your point about MAD, but it's looking forward to the endgame.
if we don't do something to stop him now, he'll just get more and more emboldened and push further and further until we do. We've already mentioned that the west has a much better army than Russia, so he can't win that way. At which point his only option would be to ask for help from other nations. Who would support him? China would want to stay out of it and if push came to shove would be just as likely to side against him.
So then what? He faces certain defeat and he knows it's all over. Is he just going to hole himself into a bunker and shoot himself in the head? Maybe. But I think he's just as likely to mchine gun everyone in the room, press the red button then shoot himself in the head. If he can't have it no-one can. I genuinely think he's that crazy.