The Budapest Memorandum gave countries a ton of lenience, including to intervene “for self-defense” and only maintained territorial integrity for the existing Ukrainian state (defined as one without foreign military or economic coercion).
Russia justified their violation of the Budapest Memorandum by pointing to US intervention and funding in Euromaidan to overthrow the existing Ukrainian government as coercion which already violated the Budapest Memorandum.
Ukraine shouldn’t have signed the Budapest Memorandum as it was. It was designed by the Russians and Americans to remove a potential nuclear adversary without providing Ukraine with any actual foundational support or protection. It was a shit deal without security guarantees and with enough loopholes to, well, drive an army through. The new deal would have included security guarantees, which are as strong an enforcement of a treaty as anything.