To Russians, 'Nazi' is not a political ideology or a party, it's simply "Whoever is against Russia right now". Are you against Russia? You're a Nazi, it's as simple as that. If they had to contend with the idea that Nazism is a political ideology, they'd have to start calling themselves Nazis, which doesn't work in the Russian psyche.
So, when Russians say that Ukrainians are Nazis, what they really mean is "Ukrainians are fighting back when we kill their children. That makes them enemies and therefore Nazis"