This discussion may contain spoilers!
Something just occurred to me. Bigby returns to his apartment to find Colin in his chair. The Book of Fables states that Colin often wanders down from the Farm to the main areas of Fabletown.
So what if Colin is taking advantage of his appearance?
What if he sneaks down, uses a glamour to mask his obviously non-human appearance, and commits the murder of Faith? We knkw that Fables can change out of a Glamour at will: Gren does it in the bar. Colin speaks to Bigby about the house he owes him. Is there long standing resentment between the two? His presence in Bigby's apartment could be a ruse. Why would a murderer sleep in the house of the sheriff? But here's the thing: does anyone know what Colin looks like with a glamour? After all, he lives at the Farm.
Also, Colin tells Bigby he guesses he was bad to every person he met that night, but Bigby tells him he was good to Faith. In the time between Bigby sleeping and Snow arriving, where's Colin? It seems a little far fetched that he knew it was Faith as Bigby didn't know her name and that he would havr been able to escape silently from Bigby's apartment, but it's an interesting theory, and possibly a plausible way to avoid being recognised by the other Fables.
Colin's resentment at Bigby for his house, coupled with fact that he can't allow him to stay in Fabletown without a glamour (Colin probably lacks the funds to regularly get a Glamour) could be a motive for him to attack Faith, who brings out the little-seen softer side of the Big Bad Wolf (couldn't resist!). He can pose as simply being frustrated with life on the Farm, and staying with Bigby, to appear like he's got nothing to hide.
So am I a genius, or am I over reading the Colin scene by a mile?
Oh, and I apologize for any grammatical errors and spelling mistakes. It's impossible to leap around these long paragraphs on my phone.