[aguaviva]

Yatsenyuk ... who decided to seek NATO membership

In August 2014, and only after Russia invaded both the Crimea and the Donbas. In fact even after the invasion of just the Crimea (in March) his government was still playing it safe, and renouncing any intent to join NATO (Reuters, March 18):

   Ukraine's new pro-Western leadership is not seeking membership of NATO, Prime Minister Arseny Yatseniuk said on Tuesday, in comments intended to reassure Russia and Ukraine's large number of Russian-speakers.[0]

It only changed its tune -- was forced to change its tune -- when Russia doubled down on its "covert" intervention in the Donbas by sending armored vehicles with Russian military plates, and attempted a similar assault on Mariupol.[1] Which (unlike the events you are falsely describing) actually did tear the Budapest Memorandum (and the UN Charter) to shreds. And rendered whatever "defensive" objections Russia may have had to Ukraine's bloc status entirely moot.

[0] https://www.reuters.com/article/uk-ukraine-crisis-nato-idUKB...

[1] https://en.wikipedia.org/wiki/War_in_Donbas#August_2014_inva...