I think the risk is that those around Putin (regime), and Putin himself to an extent, believe that a world without Russia is not worth having. And to the regime, Putin is Russia and Russia is Putin. The question is, if there is a 100% chance that should he lose this conflict that his regime will collapse and a 99% chance that Russia could survive a nuclear war on equal footing with the west which will he choose?
Even a full scale nuclear war will not end the world, many millions, likely billions will survive. It will just be nothing like the world before.