My theory on how the ribbon decaptation possible work
Like it has been pointed out by millions of people, the ribbons are surely linked with the "my lips are sealed" spell and seemly with the "magical decapitation" too. I've read around there's some "red ribbon" tale, "a little story about a girl who wears a ribbon and when she takes it off, she's head comes off of her body", therefore I would like to present my theory on how I think the "magical decapitation" may have happened...
My theory starts with the hypothesis that the girls' heads are already severed and "opening their lips" (having them say something they are not supposed to) unlaces their ribbon, resulting in their death (by their head falling off). And maybe that's how they got really stuck into the prostitution business (neck cut not deep into debt).
I think that the killer may be more than just a murdering maniacal... I think however did kill the two prostitutes (the first one which I don't think was the "Faith" we met in episode 1...) did this to expose something really dirty that's going on Fabletown. I think that the Crooked man and his thugs are not trying to protect the killer, but to get their hands on him/her to figure out what the fuck happened. Bigby is just getting in the way and dangerously close to them. I don't think they even care much about Bigby and that's why Mary's boss didn't have her kill him, like he's more trouble than it's worth.
Note that I'm not saying that the killer is probably a do gooder, but that there's more to it than serial killing, maybe deep/serious dilemmas. What do you guys think?