from the most recent episode, Boone is the second killer, and I don't think anyone accused is the main killer: so Munsch, Wes (the dad), Grace, Chanel #3 are out.
75% positive the main killer is Hester / Lea Michele. Rewatched the pilot and Gigi proposes to Munsch to open pledges to all the campus, allowing Hester into Kappa. Hester shows up wearing dated clothes (much like Gigi). She's really into murder as she proposes a way to get rid of Chanel #2's body.
10% could be Chanel #1. She doesn't have any clear motivation to be the killer, but she has killed and is never really around when the killer is.
15% the female baby is a red herring and someone bonkers turns out to be the killer, like Denise Hemphill.