He has stated that he wants a buffer zone. They don’t want a Ukraine that will have NATO missiles pointing at them, any more than the US wants Mexico to have Russian missile pointing at it. It’s quite simple.
He has stated that that he wants unconditional surrender. He could have his buffer state in a heart beat if he agreed to pull out of Ukraine. That, however, is something that he will never do. Has he not declared the conquered territories an inalienable part of Russia? He has also, many times in the past, stated that Ukraine is not a real country. So it is not quite so simple after all.