Russia started the war in the first place because — whether you think the fear is reasonable or not — Russia has loudly proclaimed for decades that they view NATO encirclement as an existential threat, and Ukraine joining NATO would massively expand NATO presence on the Russian border, leaving basically only Belarus as a buffer. And if Ukraine took Crimea with them while joining NATO, Russia would lose access to their only deep warm-water port on the black sea, Sevastapol, a port that is economically significant to them.