Wow.. sorry for your loss. Truly I live in dread of the day I find something similar in my coop. What a horrible feeling.
Here's the thing.. if it was human, I really think they just would've taken the birds. I really can't see a person stopping in the pitchblack to gut a chicken carcass.
I'm not saying it CAN'T be.. but doesn't really sound plausable. If you are going to go to the trouble of taking the birds, why not just TAKE THEM?
I agree with the pp who suggested some sort of other animal. My cats, also, will leave weird body parts around from mice/rats/moles/what have you. I had a perfectly severed head in the yard.. the next week it was a body, w/o the head (not the same mouse!).
I have no idea what you have there for predators (first thought that came to mind was maybe one of the bigger cats: mt lions, lynx, etc, but don't know if you have them there.. and I only thought of them because of what my little cats do, not really sure what they would leave).
Dust with flour around the coop. See if this creature comes back.
Meghan