He would make more sense, if he was honest with the world and say, I want the Ukrainian territories. This talk about the genocide of Russians by the Nazi regime, or "denazification" just confirms that he has lost touch with reality.
I view the stated "goals" of Putin to "denazify" Ukraine more as propaganda, and not necessarily something that Putin actually believes. I see these "goals" as a pretext to invade. Putin wanting to expand Russia's territory is not something that will be accepted by his people, or the rest of the world, and even his current supporters would likely oppose his actions.
Since Putin is apparently now a crazy old man with a Napoleon complex, this horrifying thought occurred to me: If Ukraine resists for many weeks, why wouldn't Putin use a nuclear weapon against them? It'd be like the situation with Japan and the US in WWII: Putin can nuke a couple of Ukrainian cities (probably in the western half) and say, "Surrender unconditionally or I'll nuke a couple more." And he could just continue nuking cities until Ukraine capitulated. NATO wouldn't respond with direct military action, Ukraine doesn't have nuclear weapons of its own, and sanctions are already nearly maxed out. I don't see how Putin has much to lose with this strategy (except that it might horrify his direct subordinates or population enough for them to oust him), and it's a sure-fire way to win Ukraine.
I think nukes serve more as a deterrent than an actual weapon of war. In other words, the threat of using nukes is going to be more effective in winning a war than the actual use of nukes.
There are many risks to using nukes against Ukraine.
You mentioned that (trying to) using nukes may result in Putin's subordinates ousting him from power. I think this is a very real threat. There are many reports that most Russian soldiers do not really want to be fighting Ukraine, and were misled when they were initially deployed. Using nukes against Ukraine would remove any potential pretext that Putin is trying to save innocent lives, and those who allow Putin to remain in power may decide that Putin staying in power is too dangerous.
Depending on how the nuke is delivered, other countries may detect that a nuclear attack is about to take place, and it may not be clear who the attack is against. Putin has vaguely threatened nuclear repercussions to those who interfere with his war, and much of the West is supplying Ukraine with weapons. It would not be outside the realm of possibility that Putin could prepare to nuke Ukraine (or actually do so) as a means to hide the fact that he is (also) about to launch a nuclear attack against another country (or countries). If a country makes a (mis)calculation that they are about to be hit with a nuclear attack, they may launch a nuclear attack against Russia. Alliances have been betrayed in world wars, so even China may reach the conclusion they are about to be hit with nukes.
Russia's last real trading partner is China. Currently, China is supporting Russia, in large part because they want to take Taiwan. I don't know if China is comfortable continuing to support Russia if Russia starts using nukes. There would be the potential that China could face sanctions if Russia were to use nukes and China were to abstain from imposing sanctions against Russia. China facing strong sanctions would risk an uprising by the Chinese people, and I don't know if China is willing to take that risk for a war that they don't stand to benefit from. China's economy is already in a bad position due to its housing bubble popping. China does not need to take Taiwan immediately, and China knows how to play the long game to get what they want without going to war (see what China has done to US colleges, and how that has negatively affected the US).
The war is already unpopular in Russia. Using nukes against Ukraine would result in many innocent lives being lost. Currently, there have been a fairly small number of civilian deaths in Ukraine. There is the risk that the destruction from a nuclear attack would be sufficiently bad that the Russian people would oppose Putin in large enough numbers that it would be impossible for him to stay in power.
edit:
I think what might actually result from all this, is that Russia will be broken up into smaller countries, stripped of its nuclear weapons, and
heavy sanctions will be imposed on them to pay for all the damage they have caused. Think Germany after WWI.
Who's going break them up? Whoever controls Russia's nuclear weapons can stop any effort like that by threatening to nuke the aggressor, so you'd have to have the leader of Russia (Putin or his successors) voluntarily give up their nukes, etc.
If the sanctions are sufficiently painful, those in power in Russia may be willing to negotiate to have the sanctions lifted, and may be willing to give concessions in exchange for the rest of the world trading with what is now Russia again. I do think what af_newbie is probably a stretch, at least as long as China is willing to trade with Russia.