Depending how mental they actually are, it would be a pretty ******* brutal 4d move to allow Ukrainian forces to mass in certain areas and then drop a nuke on them to wipe them all out in one swoop, just depends how far the Russians are willing to go in destroying their image, when most of the world hates them, does it matter if the whole world hates them (although I wonder if the likes of China, NK would see it differently effectively opening the doors for them to use nukes themselves on countries they dislike)