Putin, like many other Russians of his generation, never got over the collapse of the USSR and his greatest ambition is to restore Europe to its post-World War II state, and that is what he has been working on for the past twenty years (with the abundant help of his poodles, such as Merkel, Trump, Orban, Vucic...). He reckoned that Ukraine would fall quickly. Then Moldova would follow, because of the renegade Pridnestrovian republic, then the Baltic states because of the connection with the Kaliningrad region, in the meantime, Hungary and Serbia would make an agreement with Putin in exchange for parts of Romania and Kosovo (and free gas), Republika Srpska would declare independence, which would be the start of a new war in Bosnia, etc, etc. In the end, Putin would restore Russian influence to half of Europe, as close as possible to the line Szczecin - Trieste. Only naive people can believe that Putin would stop at Ukraine if things were going the way he imagined. Europe defends itself in Ukraine. It's sad, but such is the fate of small nations. The big ones always balance their accounts through the small ones.
On the other hand, this in no way absolve Ukraine from responsibility for the restoration of Nazism, nor Europe for not paying attention to it, to be clear.