Ukraine

Destabilizing Putin will only fuel the ultranationalist takeover.

We have no idea what will happen in a post-Putin Russia. I am under no illusion that it will change for the better and suddenly become truly free and democratic, nothing in Russian history suggests that will happen, but the actual scenario is a complete unknown. Also your fears about ultranationalist takeover is a bit late, that already occurred with Putin more than a decade ago.

As it stands now, any casualty inflicted on the Russian army and the destabilisation of Russia is a good thing, because it helps undermine the ultranationalist imperialist project which Putin has dragged Russia into. Despite Russians being actual human beings, some of those human beings are actively engaged in an invasion and planned genocide of a neighbouring country. There are no "both sides" to that, that is a fact existing in reality.

And anyone not deserting or surrendering to Ukraine are legitimate targets in the effort to minimise the damage they are currently doing to the victim country. Russia is systematically and deliberately bombing civilian targets each and every day. They are torturing and killing POWs and civilians, and it will continue to happen for as long as they are allowed to engage in their war of aggression. Stopping this by eliminating the invading soldiers one by one, and destroying their equipment piece by piece is the only way, because neither Putin nor Russia will willingly back down from their invasion and grand plans of their Greater Russia.