If an astronaut killed another astronaut on the moon, would he be arrested when he got back to earth?
A lot depends on the nationality of the spece killer and whether his/her country is a signatory of the Outer Space Treaty. See http://en.wikipedia.org/wiki/Outer_Space_Treaty
Anyone suspected of a crime would be detained by the MOON POLICE, and wouldn't be allowed to return to Earth until after a trial. http://en.wikipedia.org/wiki/Project_Horizon