diplomatic immunity should NOT be able to be used for criminal actions.
Well, I suppose the idea is that if you are a diplomat in, say, North Korea, they cannot bust into your hotel and drag you off to a gulag for some fake "crime" and then essentially ransom you back to your government for concessions of some sort. Which is all well and good until the diplomats figure out they can commit crimes with no consequences and the bad actors begin abusing it.
If it isn't, China will start arresting U.S. diplomats on bogus charges.