Iran was under UN sanctions at the time Meng violated those sanctions. China is on the UN security council, and as a permanent member, they have a veto. They did get a vote, and they voted yes.

Are you somehow not aware of this? They were a founding member of the UN, and have been on the Security Council since it's beginning. They have abstained or voted yes on every UN action that involves the SC (international peace and security).

Meng violated additional US laws... which she is only subject to because she was operating in the US. Just as a US citizen is subject to Chinese laws when operating in China. Surely you're not suggesting that foreigners should be allowed to go into another country and violate their laws without consequence.

Are you somehow not aware of this?

The PRC became a permanent member of the Security Council (with a veto) only since 1971. Taiwan was the permanent member from 1945-1971.

Meng is an individual person, which not represent the company or the organization. If US has a war with China, should China legal to kill any US citizen in China?

Just as a point of clarification, Chinese have no obligation to comply with American sanctions, unless they’re using the American financial system.

Right. You are free to trade with Iran, as long as you don't mind disconnecting from the US financial system. Or you try to set up a shell company that does the trades but keeps the parent company from violating the sanctions (to be fair, Chinese companies learned how to do this from American companies).