Since i just saw a discussion in the steam forums...the clickable area to bring up the menu selection for corpses is very small. Fishmen are sometimes really hard to designate. This was much better with the area selection of 0.54 and before.
Ditto this one. The resolution / work around is to drag and click over the area around the corpse. You'll get a little popup with some corpse handling options. I just wish I would have known that earlier in the game.