The guesses for why each person is the killer, along with proofs, both direct and circumstancial are accpted.
I think that Kam is the killer personally. When Sheri was murdered, both Kam and Adrianna were outside. The killer was outside as well when he shot the slingshot at Sheri. Another hint was in a scene in which Ulysses and Kam were discussing the slingshot. Kam then said, "He killed her with a slingshot?!" He used the pronoun 'he' instead of 'the killer' therefore developing a theory that he might be the killer. It may have been on honest mistake, or a subconscious slip up that points to him being the killer.
I think Melina is the killer because she is always over exaggerating for everything. She cries, screams, talks hoplessly, and acts innocent all the time. She is always clueless when it comes to solving the murder. She has never solved a riddle, though has recieved a spare card every week but one . She never is the top guess nor the bottom guess hoping that the spotlight is never on her. She wants everyone to like her and think of her as the "innocent little girl". When it came to seeing the foot prints, she kept repeating that they were Geno-sized footprints, though she stuffed the size 13 shoes with paper so it fits her. I don't see a way she can't be the killer.
I think that Cris is the killer because in the episode that Geno was murdered, Cris mentioned that she grew up with guns and Geno was shot with a gun, besides that she has never got a scared card. She has also solved 2 riddles which is one more than everyone else, disproof: Kam solved three riddles, which was more than anyone could do. When Dana and Sasha died and everyone was trying to solve the riddle, she looked inside the shoes, how would she know to lookin there, Cris is the killer.
I think Lindsey is the killer because The Killer was riding a black horse and only Lindsey and Ulyses rode one! Lindsey also seems to be least likely to be the killer, like the other people have little things that point them to being the killer, Lindsey seems to have none, (besides that tiny one above) and if the killer has killed so many people already without leaving a trace, why would Lindsey give everyone a reason to think it her.
I personally believe Lindsey may be the killer. She mentioned being a pastor's daughter, and two of the murders contained a bibical clue.