Russia is a very flawed democracy, not a dictatorship. Putin has to face the electorate on a regular basis. I'm not arguing against the right to fight for freedom, just asking if it's worth the huge cost in lives. How do you know that "Russia will genocide Ukraine"?
Because they're already fucking doing it in the places they occupy. As for Putin, yes, he is a dictator in all but name, unless, of course, you believe Russian elections have any sort of credibility.
The only just peace is one where Ukraine regains every inch of occupied land and Russia is held to account for its crimes (reparations, war crimes trials, an acceptance of collective responsibility and, ultimately, decolonisation). Otherwise you're just rewarding aggression and laying the ground for more aggression in the future.