On February 4, Russia and China vetoed the Arab League's one-sided Syria resolution (SC/10536). It illegitimately called for Assad to step down.
Under international law, no nation or combination thereof, may interfere in the internal affairs of others, except in self-defense if attacked.
Russia and China want replicating Libya avoided. Passing SC/10536 risked giving Washington, NATO partners, and rogue Arab League allies responsibility to protect authority to intervene.
As a result, this unholy alliance circumvented SC authority for General Assembly passage of essentially the same text. It's non-binding but sends a message.