I'm pretty sure Keiran's the killer. I know I have a spotty record of predicting these things at best - I mean, I called Piper in Season 1 way earlier, but Harper's Island and the Mentalist had me completely fooled. Although I maintain that Harper's Island cheated pretty severely - although I have no intention of relitigating that here.
Alright, so here's my reasoning-
At the end of Season 1, we were left with a question - who attacked Will and Piper, if Piper was the killer? Audrey obviously knew something, but unless the show was massively cheating, she wasn't the second killer, since she is tiny, and he was absolutely not-
Now, unless the show wants to cheat its audience, the killer has to have been someone who was introduced during the first season. So by height alone, Keiran's basically our only suspect.
But could he have killed everyone in Season 2? Let's check!
Jake: Goes out to boondocks to prep burning down the mayor's subdivision.
Where was Keiran: Supposedly sleeping on the couch in his living room. Of course, Emma didn't notice him getting out of bed with her and leaving the room, so this isn't much of an alibi.
Motel Clerk: Stabbed to death in motel while Emma is lured to find the body.
Where was Keiran: Who knows? He was absent for almost all of the episode.
Emma Attacked in Woods: May not have happened, but probably did.
Where was Keiran: Possibly sleeping in Emma's bedroom, but there were no witnesses. So he was definitely in the area.
Emma Attacked During School Lockdown: May not have happened, but probably did.
Where was Keiran: In the school, which the killer had to be, and notably absent from the library in the scene directly before the attack.
Gang Chased at Empty Fairground: The killer just menaced them here, making no real attempt to hurt anyone.
Where was Keiran: At the empty fairground. He runs up to the gang from the direction the killer was last seen, and claims not to have noticed the killer.
Sleazy Teacher Mutilated: It's not clear how the killer found out about where the sleazy teacher was - maybe phone surveillance?
Where was Keiran: No alibi. Last seen leaving Emma's house in his pickup truck, which has more than enough room in it to conveniently store a body.
Teacher Thrown Down Stairs: This is a legitimately bad-ass stunt that we should all enjoy together.
Yeah, that was just brutal. Although it's not clear how the killer could have possibly gotten Sleazy Teacher's body into and out of the building in broad daylight without being noticed.
Where was Keiran: No alibi. Other than a scene at the beginning where Emma yells at him for trying to save her life, he is not in the episode. Also, Emma is the worst.
Model Home Burned Down: While Emma is on her date with Evil Cousin, the killer sets the house on fire to keep them from discovering the bodies upstairs. Presumably the killer knew this would be a good place to hide bodies because Jake was planning to burn the house down at the behest of the mayor, meaning zero security.
Where was Keiran: See above - it's the same episode. And Emma really is the worst.
Keiran Attacked At Carnival: Yeah, this makes the whole theory not look that great. That being said, there are things called selfie sticks, and it's actually pretty weird that he's looking that far off camera - as if the killer was like four feet up and to the right of where they were aiming the phone. Why, it's almost as if using the selfie stick makes it hard for him to gauge exactly where his eyeline should be!
And this is the point where I've officially gone too far down the rabbit hole. Seriously, selfie sticks? He could just have yet another partner.
Where was Keiran: Doing all that stuff in the above paragraphs. Also, it's a risky play to almost get shot by cops, but Keiran was in a rough situation, with Emma pulling away from him and spending time with Evil Cousin. So he's got motivation, and could easily have pulled the plan off.
Horrible Teen Stabbed At Party: Haley, who hates Emma, is super-confident that the person she's seeing couldn't possibly be the killer. Also, that person suggested she set up the party. Obviously she would be super-psyched if Keiran was cheating on Emma with her, given her feelings about Emma, so her manner is completely explained by the situation.
Where was Keiran: At the party. He seems to call the police (although who knows if he actually did), then gets lost in the crowd as Emma goes to confront Haley. After Haley is stabbed to death, he appears back in the crowd, looking for her.
Noah Stabbed And Kidnapped: The entire episode is devoted to Emma and Audrey's attempt to rescue Noah and Zoe. It doesn't go great.
Where was Keiran: Not in the episode.
Killer Steals Emma's Dream Journal: The killer walks confidently through the house, as if super-familiar with it. There's a questionable moment where he opens the mom's door, but it can be read either as getting the wrong room, or just checking to see if she's asleep. More importantly, the killer is there for the dream journal specifically as part of the 'frame Emma' plan, and therefore already must know it exists.
Where was Keiran: No way to tell. He was home a few minutes later to receive Emma's phone call, but there's no reason to suspect that's a long journey, especially since Evil Cousin was also there.
The killer does tap a photo of Keiran and Emma with his knife-
Which can be read as a threat, or Keiran being kind of proud of how well this is all going.
Mayor Stabbed With Pitchfork: The entire episode is devoted to making it seem like Evil Cousin is the killer. They find evidence that he was in town during the first spate of killings, he goes to the mayor's house and threatens him, Keiran finds Audrey's missing letters to Piper under his mattress. It looks bad for Evil Cousin, and good for Keiran. Which is why this is the episode that tilted me pretty far the other way.
So let's lay out the evidence.
Keiran overhears Evil Cousin talking about his plans to blackmail the mayor (he's talking to his mother), and sees the blackmail documents.
Keiran finds the packet of letters hidden under Evil Cousin's bed. Naturally these must have been planted, only to be 'discovered' for the benefit of Audrey and Emma, who are on facetime while he searches. It's not incredibly likely that the killer - who would need some kind of a workspace to hide his costumes and set up his murdering tools, would have hidden incredibly incriminating letters under his bed, of all places.
The Killer texts the Mayor and lures him to his death, using the threat of revealing the documents that were stolen that afternoon. The Mayor is only able to be lured because of the robbery, so the killer must have known that it happened. At this point, only three people know about the Mayor's secret blackmail file theft. Keiran, Evil Cousin, and Drunk Aunt. Keiran pointedly does not tell Emma and Audrey about it at the car, even though it would go a long way towards proving just how sinister Evil Cousin is, and bolster his case for going to the police.
The biggest clue, however, comes in the Killer's plan. Extreme lengths were gone to in order to frame them for the series of murders. This culminates in having them find the Mayor just moments after his brutal stabbing. The only way the killer could have arranged this is if he knew where everyone was going to be - only Keiran was in a position to have all of this information. Emma and Audrey decide to go to the pig farm after he leaves, but then Emma immediately announce that he'll text Keiran with their plans as she gets into the car. Soon after, the mayor is texted and dispatched out to the pig farm. So either Keiran is the killer - as he's the only person who could have contrived this meeting, or the show is asking us to accept this astronomically unlikely coincidence.
The final part of the plan involves having the police show up and find Emma and Audrey standing over the Mayor's body. Keiran is the one who calls the police. So either the killer got incredibly lucky with independent decisions made by two different people, or Keiran is the killer, and the plan went great.
Where was Keiran: In the vicinity of the Pig Farm - he drives up literally minutes after Audrey and Emma are arrested.
So that's the argument, and the evidence. Even being incredibly critical of my arguments, the only one it's doubtful Keiran could have done is killing Jake - in all of the rest he's the best suspect in the show. Evil Cousin is, of course, second best, since he wouldn't have had any trouble killing Jake, but he couldn't have arranged the conclave at the pig farm in episode eleven.
Am I right or wrong? You can check right now by watching the final episode - I'm going to be doing that tomorrow, after which I'll put up a post-mortem offering my thoughts on the season as a whole, and either gloating or apologies depending on how things shake out.
See you there!