They start out slow-mo then just do this very quick animation that is much faster than a normal animation/real finish would be. I suppose it was done so you wouldnt be caught in a long finisher while getting killed by other enemies, but they could have just gave you like 90% damage reduction and left the animation at a normal speed; much like Fallout 3/New Vegas.

Also.. this game will be awesome with mods. It is good now.