2. No point of the Budapest Memorandum implied that the signatories had to defend Ukraine.
Paragraph 4 essentially states that signatories have seek UN Security Council action if Ukraine became the victim of aggression or a threat of aggression involving nuclear weapons.
Ukraine isn't victim of aggression involving nuclear weapons, and even if it was, the signatories merely said they would escalate the matter to UN's security council.
Another paragraph had signatories (China, France, US, Russia, UK) to refrain from using force against Ukraine. Russia has violated this point.