This purported promise is a myth, see my comments here [1]. (TL; DR No treaty or public comments. Russia, on the other hand, signed a treaty promising not to invade Ukraine when she denuclearised. Talk of broken promises is a red herring.)
Russia has a realpolitik interest in creating buffer nations between it and the West. What it's doing--invading Crimea and now threatening to take the rest--doesn't further that goal. It does, however, serve Putin's political agenda.
This is what Russia is promoting but it's a gross exaggeration. When the Berlin Wall fell, The Secretary of State at the time, James Baker, threw this out there to Gorbachev in early treaty talks as a "what if?", but others in US gov. said absolutely not, it never made it in the treaty, and Gorbachev never pushed for it. Gorbachev himself said that no promise or treaty was violated, but it was against the "spirit" of the assurances.
https://www.brookings.edu/blog/up-front/2014/11/06/did-nato-...
Sorry if I am destroying your good, nuanced conversation.
...and then I see comments like yours that just seem to want to make Russia into this mustache twirling always evil villian and to me its people like you destroying hopes for good conversation that seem like the real villians.