Excellent question! In order to do the subject any justice, we need to explore fascism, Russia, and Ukraine, then circle back.

1. Fascism

In general, what is fascism, and what causes it? Fascism is, essentially, violent reaction to assert and maintain bourgeois hegemony. It rises in reaction to socialism, and to capitalist decay, coming from small business owners and sole proprietors above all else (who are pushed downward closer to the proletariat, their class position is precarious). Anti-communism is core to fascism, and usually nationalism is also core, which leads to attacking "out-groups."

Capitalist decay breeds reaction. It's also helpful to understand that as imperialism decays, austerity is forced inwards to cover costs. As profits from plunder go down, safety nets are trimmed so that the working classes absorb the brunt of the impact while the imperialist bourgeoisie stay on top, relatively unscathed.

2. Russia

The modern Russian Federation is a dictatorship of capital, sort of like a social democracy. Large state owned enterprises are leftovers from their soviet heritage, but the economy is thoroughly controlled by the nationalist bourgeoisie, the "oligarchs." Capitalism was devastating for Russia, so the second largest faction behind the nationalists are the communists, in the CPRF (not the pro-west liberals).

This causes the nationalists to try to appease the communists seeking a return to socialism by trying to claim soviet heritage as Russian heritage, not socialist heritage. It's a balancing act, where communists are gaining in power and the nationalists are losing power to them, but they are still far apart in terms of total power.

The nationalists gained power by kicking out western imperialists that looted the country in the 90s, but now they are losing their firm support amongs the public gradually to the communists.

3. Ukraine

Stepan Bandera was a Ukrainian nationalist and Nazi collaborator. With the dissolution of the soviet union, these nationalist groups began to solidify, such as Azov. Capitalism brought rampant destruction and decay, and bred reaction.

Ukraine is one of the poorest countries in Europe, and one of the most devastated by the dissolution of socialism. This became a ripe breeding ground for fascism. This is why they are tearing down soviet monuments, and upholding Stepan Bandera.

4. Circling Together

The far-right in Ukraine oppose Russia, yes. They do so as part of their anti-communist stances, opposing their own soviet heritage and despising Russia for refusing to do so. They also have connections to the western imperialist bourgeoisie, who have been supporting them for the same reasons they support Israel, as attack dogs. Within Ukraine, the Donbass region was historically Russian, and as such is also targeted by the Banderites in Kiev.

The 2014 Euromaidan coup allowed the Banderites to take power, oppressing Russians in eastern Ukraine, and allowing the west to imperialize and plunder Ukraine in exchange for personal wealth. Essentially, corrupt compradors were installed, modern Ukraine is a gangster state more than anything.

Leftover unresolved contradictions from the disollution of the USSR were leveraged by the west to spark the conflict and attempt to force Russia into opening their capital markets for foreign plunder as they already succeeded in doing in Ukraine.