There's always the possibility that the rumours about Putins' failing health are true. If he knows or believes his days to be numbered, he won't want to be remembered as the man who failed to take Ukraine.

He also may believes that a nuclear threat will be enough to start peace negotiations which will allow Russia to hang on to some bits of Ukraine and allow him enough leverage to turn the rest into a client state. But it's too late because nobody will ever believe he's negotiating in good faith.

That said, he may already have put a nasty crack in the EU!