There is no binding defense treaty except the one Russia signed.

Russia remains wholly responsible for every death on both sides. Russia signed a binding treaty promising to respect Ukraine’s independence and sovereignty of their existing borders (violated), refrain from the threat or use of force (violated), refrain from economic coercion (violated), seek immediate Security Council assistance should the signatory become a victim of aggression (violated), refrain from use of nuclear arms (threatened), consult with another treaty member should questions arise regarding those commitments (violated when they violated all of the above).

Russia has a literal obligation to shoot itself for invading Ukraine that was signed by the current government. It’s not some cheap “Well we’re the successor of the USSR” as if California takes over the USA’s international slot following a fifty state breakup.