Ukraine wasn't "set to station nuclear missiles" either, unless we're in pre-crime territory.
They wouldn't need to be-- yet-- for it to be the case that they very nearly may as well be, from the Russian perspective it only being a matter of time as subsequent developments would be out of Russian control. NATO membership implies mutual defense with America's gigantic nuclear arsenal, and unlike the Soviet Union, the United States has never pledged a no first use policy (not that I'd necessarily trust it if it did...) NATO has callously disregarded Russia's security concerns over Ukraine's aspiration to join NATO and there is no particular reason to think that NATO would suddenly develop an interest in the issue
after guaranteeing Ukrainian mutual defense. Nuclear missiles stationed in Ukraine would be absolutely unacceptable in a situation in which the United States and Russia are the pillars of an international order based on the mutually assured destruction of both; it is simply too close for it to be reasonable for Russia to be able to figure out whether they're under attack in the case of any anomalous radar or other intelligence data, so you would virtually guarantee a Russian launch by accident. And the potential for nuclear missiles to be stationed in Ukraine while Ukraine is protected by mutual defense treaty with the United States nuclear arsenal means that Russia, by the time Ukraine was a part of NATO, would have no recourse other than an escalation to probable nuclear war or the United States and Ukraine unilaterally deciding never to put missiles in Ukraine (NATO membership would naturally obviate any compunction to abide by the Budapest Memorandum, because what would be the point? Some minor concessions on economic pressure that Russia was not apparently respecting anyway so far as I can tell; the Budapest memorandum would be superfluous for Ukraine given NATO membership). That is not an acceptable place to be in. It would be national security and the fate of humanity relegated to prayer.
Or to put it in a more visual manner:
Ukraine has neither nukes nor NATO membership -> Ukraine has NATO membership -> Ukraine has nukes
... is a chain that can only be stopped at the first of those three stages without an unacceptable risk of nuclear war. So if Russia is to stop the threat of Ukraine having both nukes and NATO membership, it has to do so before it has a NATO or any other NATO-like security guarantee. Granted, none of these steps necessitates the next step. But apart from the first, they completely nullify any Russian say in the matter of whether the next step is taken. Now, you can argue that Russia was early, perhaps, in abandoning hope of a diplomatic resolution without such a largescale invasion because much of NATO seemed not to want Ukraine to join anyway. On the other hand, it has been literal years of both NATO and Ukraine ignoring Russia's entreaties on this topic, and, while Ukraine didn't have NATO membership, it was receiving "lethal aid" from some very high profile NATO members in order ostensibly to fortify itself, another process that, allowed to go on long enough, could very well result in Russia having no leverage capable of stopping nuclear weapons from being placed in Ukraine. So there you go, that's the Russian rationale that actually makes good sense (not to be confused with the nationalist rhetoric which largely doesn't, or at least isn't at all persuasive to anyone but people like Navalny).
Can you blame Putin for not proposing a trade of Crimea for the desired guarantee of Ukrainian neutrality as was apparently an idea in Russia in 2014? In my opinion, yes. But that ship seems to have sailed.