Putin doesn't see Ukraine as a sovereign nation separate from Russia.
His genocidal aim is to wipe out Ukrainian language, culture, and church and to murder all Ukrainians that do not accept being Russified but it's also OK for him if they flood Europe and destabilize it with a refugee crisis that they can't handle. Remaining Russified Ukrainians are good for slave cannon fodder for future imperial ambitions beyond Ukraine. Putin isn't crazy, but he is evil.