To say they’re only killing Nazis is a bit of a trolling oversimplification, and I wouldn’t essentialize Russia’s motives down to only liberating the people of the Donbas, but “russian troll” is RussiaGate BlueAnonsense.

As to whether Russia is doing imperialism, I’ll copypasta myself:

Honest question from a non-communist, based on your reply here. Does one need to support Putin to be a Marxist?

In a word, no. In a few more words, support for Russia (not Putin, as historical materialists don’t subscribe to great man theory) is only a partial, temporary, tactical one, in the context of imperialist liberation. Russia is still a capitalist state, though, so it’s a two stage strategy: first liberate colonized bourgeois states from colonizer states, and second revolution within those liberated bourgeois states.

Russia is an interesting case: it has already liberated itself from the post-Soviet “shock therapy” neocolonizers. This occurred during Putin’s administration, which is why he is especially hated by the US. So now the support for Russia is in the context of keeping the colonizers from recolonizing it, and supporting Russia to the extent that it helps other states liberate themselves. But Russia isn’t trying to “liberate” Ukraine, at least not all of Ukraine. It’s trying to resolve the genocidal attacks on the people of the Donbas, and it’s trying to resolve the imperialist military expansion at its border.

Robert Parry, 2015: The Mess that Nuland Made

Jeffery Sachs, 2023: The War in Ukraine Was Provoked—and Why That Matters to Achieve Peace