This discussion may contain spoilers!
There are lots of speculations. I think the guy who did this is not still added to game. But Tweedle Dee and Dum, Their Boss, Headless Horseman and probably Red Head guy are somehow connected.
Also I would like to mention a rhyme that is in Episode 4 in achievements. When you take all the titles of each achievement ''Complete chapter 1-5 of Episode 4'', the rhyme will come out:
''Once Upon a Time - There Was a Wolf - Who Ruled the Land - He Was Much Feared - But Soon He Mended his Evil Ways - And all Were Happy''
Im not good at english but I think that mean that Bigby is going to be the bad guy or he's going to get better and he will become a good guy.
I made a list of my suspects:
Crane: He is probably not involved in it. He's just angry becouse Bigby thinks he's the main suspect. Killer can't be revealed at the first episodes.
Tweedle Dee and Dum: They were snooping around every crime scene. They were looking for something. Maybe for clues that could lead Bigby to the killer. What if the killer is their boss?
Headless horseman: At the EP.1 when you examine Faith's head cut Bigby will say ''Strange cut, something with a magic attached to it'' Horseman could just swing this sword on his horse making a circle shape but he could kill Lily becouse she was killed in her bed and she was killed by Crane (probably).
Snow: She's getting more angry than before. She's hiding something that we don't know. Also episode 4 ''The Sheep's clothing''. That mean that maybe someone that we think is 100% innocent will be a traitor. I mean, snow that looks really sweet and normal, can be somehow releated to something.
Why won't we just look at Bigby as himself: I have no clues from playthrough but from achievements again. ''This house of Straw - Complete Chapter 2 of EP.5'' (Colin is on the image) - Will something happen with Colin and Bigby's house. Will Bigby kill Colin like in a story?
''My last Cigarette - Complete Chapter 4 of Episode 5'' - Will Bigby die becouse he is the killer and he will have final battle to his death?
''Happily Ever After - Completed Episode 5: "Cry Wolf" '' - After Bigby will die everybody will live happyily ever after.
Bigby is also the one of the fables that kills everyone, he's the wolf. The title of the game ''The wolf among us'' maybe like ''The killer among us'' At the end of Episode 2 Bigby look like a wolf. He looks pissed off. But why. They will make a story like in Heavy Rain. One of main characters will be the murder.
Red Head guy: He's in every place where crime will have a place. He was acting strange when you met him with snow at the hallway. But also I noticed a bug or maybe it's how it's supposed to be. When you are on your way to TripTrap, Red Head guy is driving a cab. But after looking at the place and snow saying ''You should handle this one'' you can notice the same guy in the alley also with Gren that's in the bar at the same moment. Maybe this is that Glamour mentioned in EP.2 to make fables look like another but I think this only concerns girl working at the strip club