Look: the Russian invasion sucks for everybody involved and in particular the Russian and Ukrainian civilians. War might be good for many things but at the very least it is not obviously good for politics. I think it’s not controversial to expect that Russian leadership would have much preferred to develop influence through significant mutually beneficial relations with its neighbors.
What is to be done however if at some point a third party with infinite resources is those neighbors to an impasse on topics of national security to Russia? And after this those third parties start arming those neighbors with advanced weapon systems? Exactly what do you expect the US would do in a similar situation?
People wishing that the war would stop are not simply siding with an invader or a bully. They are being practical and recognizing a legitimate grievance of the Russians. Unless your position is that we live in a world where nations are unequal wrt security expectations you have to acknowledge the reality that the Russians have a legitimate claim to be upset about.
What's to say Russia won't wave the nuke card again, and again, if it worked the first time?
Also, do you ascribe any agency to the people of Ukraine? If they want to resist Russia, let them, I say.
And of course it's not out of altruism. (Well, some of the icing on the cake may be.)
It's because Russia is changing the status quo. That's a threat to the US. (And many other countries!)