I think he's possessed.

Just started black flag, and on the mission to free the pirates, I killed one of the guards and dumped his body in the ocean -- or so I thought. I went about my business distracting other guards until I returned and found that the guard I killed before started to magically float. Obviously some black magic there.

I then proceeded to re-attempt this glitch with more bodies, hence the sacrifices.

You'll get to see his lovely face close-up near the end of the vid. ;)