Whether it was an accident or murder it happened on board, I'd guess the body weight was too much for him to lift up through the hatch so he cut her up into more manageable pieces. Naturally this would have made a bloody mess in the sub so the easiest way to clean up was to scuttle it.
Sorry if my description is
too descriptive.
Nasty either way!!!
My heart goes out to her loved ones, may she rest in peace.