That said, I don't think even the Russians thought this war was about "De-nazification" despite that being the official line. Their focus has been on taking over the Donbas region and securing utilities for Crimea (another stated purpose of the war).
Also, it's worth pointing out that we in the west think of Nazis as primarily being anti-semitic, but Russians think of them as primarily anti-Russian, so claiming to be fighting Nazis is a good line to get popular support in Russia.
They absolutely don't justify the invasion of Ukraine and are a smokescreen, but that doesn't mean that they aren't problematic. There is a reason the US blocked military aid to them in 2018.
Meanwhile, there were stories about the Ukrainian rampant Neo-Nazi problem right up until the past few months when everyone is now suddenly supposed to be okay with the idea.
Remember, the Ukrainian Nazis had their own SS divisions (around 90k troops) and butchered countless Jews, Poles, Russians, etc. The US snuck out the Nazi leadership ahead of Russia's takeover because the US had made a deal with the devil in an attempt to counter the USSR under Stalin.
While the Nazis were punished and reviled in every other country you can name, they were never purged or hated the same way in Ukraine where they wrapped themselves up in Nationalistic "freedom fighter" rhetoric. If they ever gain power, they'll be just like ISIS or Nazis of old going around butchering dissenters and undesirables of all kinds.
The enemy of my enemy isn't necessarily my friend.
Propaganda is used heavily by all sides and the only thing you can do is recognize that and hopefully see through it.
What kinds of reasons (valid or invalid) is Russia giving? Are there actually nazis in Ukraine?