I have kept many emeralds and I have never had any issues... The males do tend to be a bit meaner... if you have a male, it will grow large, quickly and it will have orange "hinges" just behind it's large claws. It's shell will also be very shiny and it's legs will have really long hair. The females are smaller and sage like in color.
The females tend to stay in the nooks of rocks, where as the males that I have had tend to be right out where you can see them. And they can get aggressive when defending territory. I have never had them kill anything though.
More than likely, the clown was sick and the crab was doing his job by cleaning up waste.