I have seen plausible evidence for three possible killers (listed in order I am leaning towards).
1. Lindsay: daughter of a pastor, religious references in several episodes (bible passage in first eps, St. Agatha medallion in second eps), the killer mocked her laugh when she was rewarded for doing the best, the killer has not mocked any other contestant
2. Melina: Portrait in Rue Manor that is shown numerous times has striking resemblance to her, reference to Iron Maiden in last episode and this is a Marvel character who's alter ego is Melinna, references to the sky (blew her sky high, look to the sky) and she is a flight attendant
3. Geno: I read on another forum that the killer will wear red in every episode. This past episode he had a red bandana hanging out of his back pocket and on ABC's site in the clue picture's he is the only one wearing a red blindfold (everyone else is wearing white)
Now there are lots of other reasons I could argue any of these to be the killer but going off of concrete evidence only.