I’d say to scale down said debates to not tank your relationship over that. if the debate is inevitable, the “didn’t kill enough” depoliticize the discussion, and to someone outside our circle you make you look like a lunatic, so the best approach imo is bringing the discussion to context, like the other comrade commented quoting losurdo, recognize the repression but putting into the context the political situation soviet union had at the moment, surrounded by reactionary forces including the assassination attempt on lenin, to at least not end the discussion looking like stalin was a monster that killed people for shits and giggles. critic without autophagy.
capitalism is so marvelous that applies it inversely, one part gets jobless, the other work like fuck.