Say one astronaut murders another astronaut in or around the ISS, what would happen? Are there any protocols or instructions about what to do?
Calling the space police does not seem like an option.
There are two things about such an action that must be considered: what happens on the station itself so that the crew can protect themselves, and what happens once the crew returns.
To the first part, NASA does have a policy in place to handle an out of control astronaut. This article references but does not provide a link to the protocol. However, it's just about exactly what you would expect - subdue the astronaut, bind their hands and feet so that they are no longer a danger, and get back to Earth. Use tranquilizers if needed.
This all assumes that the other astronauts have the capacity to actually do this and aren't incapacitated, but that's no different from on Earth. For instance, if a police officer chasing a criminal is incapacitated and can't sustain the chase for whatever reason, then that specific person is no longer able to apprehend the criminal.
Now, what about returning? Well...it's not like you can just hop in the Soyuz and come down to Earth. It requires planning, at the very least, and a recovery crew, especially if the astronauts have been in zero g for a while. Ultimately, the answer is you get arrested when you return to Earth. I suppose in some fantasy we can cook up, a foreign spy astronaut could commit the crime, subdue the crew, and somehow plan and execute a landing in their friendly nation to avoid this...I suppose.