Yep, they ran into a situation with Jason Todd as Robin where he 'may' have killed a guy and made it look like an 'accident'. Diplomatic immunity stopped they guy from being prosecuted from killing a woman and Jason wasn't having it.
Frankly that's the kind of thing that legit should have happened to Joker decades ago. Any random cop or Arkham intern could finish him off and just throw up their hands and say 'He was gonna kill me' and totally get away with it. Probably get a raise and a parade.
Batman can still keep his hands cleans, but the fact that EVERYONE lets Joker get away with everything is mind boggling.