From the outset Russia's war of aggression sets it outside its obligations under the UN Charter, the Budapest memorandum, and Geneva Convention. But once annexation is announced by Russia, the rest of the world has to decide what to do about a violation of the most central proscription in all of international law.
I'm skeptical the UN system avoids defacto collapse if it refuses to strangle the Russian regime at this point. And if the UN system ends, whether 5 months or 50 years, I uave serious doubts about WW3 being avoidable. The UN system stinks. But it's all we have, born from the ashes of WW1 and 2, to avoid it. Since we haven't done the hard work of getting rid of nukes, I'm also skeptical humanity avoids using at least one.
China and India need to show up and defend the Charter. They agreed to be bound by this obligation.