I understand this, to some extent, but I would rather learn Russian and get along with a new set of completely uncaring oligarchs, not that dissimilar to the ones we have now, than end all life on earth.
It wouldn't even end all life in the involved countries. Submitting to military aggression is also morally wrong. Nuclear retaliation is a heroic sacrifice that does good to the rest of the world by discouraging future wars.
Furthermore, it's no longer medieval times, where the citizens are personally loyal to a king who is expected to be replaced by another king some time. The invention of nationalism guarantees civilian resistance, so the only way to safely rule a subjugated country in modern times is with a brutal North Korean-style regime.
