NATO expansion into Ukraine was widely recognized by people like current CIA director William Burns as a "red line" for the Russian elite, not just Putin. The Russians have legitimate security concerns over its neighbor joining NATO, but this invasion has only further isolated Russia internationally and bolstered NATO. Russia will have a much harder time arguing against Ukraine being part of NATO now, but since that alliance has not been willing to defend Ukraine or offer it membership anyways, the best thing the Ukrainians can hope for is the non-aligned, neutral status that both Finland and Austria enjoyed after WWII.