WTF he's death!! Sewers SPOILER!

[hollowleviathan]

It's possible that the powers that bind the alien creature to the human body, and warped it, would slash it cleanly like that upon death. We know nearly nothing about them, it could be a number of things. The possessing aliens could have death rituals that involve what we saw, including having, if I recall, two left arms left at the death site.