Witch Creek Road S3 E17 Explained (Spoilers)
The episode begins immediately after the shooting of Jay. Keifer, in shock, asks Kevin, “What... What did you do?”
Kevin replies, “You heard him! He set me up!” (referring to how Jay led Kevin to believe that the police officer was infected). Keifer points out that Jay was being sarcastic, and Kevin replies, “No, look.” He points toward Jay's body, expecting to see bugs emerging from the gunshot wound. Instead, there is only blood.
Realizing that Jay may not have been infected after all, Kevin turns on Amity, saying it must be her. He points his gun at her; Keifer, in turn, points his gun at Kevin.
Keifer pleads with Kevin to put his gun away, and Kevin refuses; they shoot each other, leaving only Amity.
At this point, there's a knocking at the door; Amity turns and see Edwin. She picks up Keifer's gun and points it at Edwin.
Edwin walks toward her, calmly, and says, “You know, a part of me is sad this is finally over. I'm not a monster. These were my friends. [...] A part of me kept hoping you'd finally get your shit together. In the end, I barely needed to do anything.” Then Edwin stands aside and says, “Anyway, you can go. You're not important.”
At this point, Amity is shot. We shift and see Kevin, on the floor and bleeding, and holding his gun. Then he attempts to shoot Edwin, but the bullets have no effect. Edwin tells him, “Did you know that none of you were infected?” Then he kneels in front of Kevin. “But that's about to change.”
To avoid being infected, Kevin shoots himself in the head.
EDWIN'S FULL CIRCLE: We've touched upon the circular nature of the curves and how events tend to repeat. This final chapter mirrors the choice of Edwin at the end of Season 1. At that time, Edwin had just been hit by a car, and as he lay on the road, bleeding and dying, the main demon approached him. Edwin, not wanting to die, pointed out that there was another way, and so he chose to become a monster rather than to death.
And here, it's Edwin's turn to stand over his dying friend. And again, the choice is either death, or become a monster. But, unlike Edwin, Kevin chooses death.
KEVIN'S CHOICE: Kevin fought so hard to expose the traitor amongst them that, by the end, it was his solitary goal. He killed everyone, only to discover that none of them had been traitors. Though his choice to shoot himself could be seen as him not wanting to become a monster, it could also be seen as him not wanting to live with the guilt of murdering innocent people. Additionally, if he became infected, he'd become the very thing he'd worked so hard to expose all along.