They have no intention of stopping at the Ukranian border - and many of the bordering nations are NATO members. The RF is vastly overmatched by NATO. A desperate nuclear power will probably resort to those weapons.
In Ukraine, we have an ally willing to fight fo their own survival - if someone loans them the tools they need.
Also, the US committed to protecting Ukraine's borders as part of the 'deal' in which they gave up the nuclear weapons they already had. The RF made the same commitment - the invasion demonstrates exactly how well they'll abide by any new treaty.