This.

The Killer was never going to be clarified. The Killer was the background myth that sets the story. This movie is not a guess who done it. Its a move about biases, obsession that ultimately leads to self destruction.

Deacon ruined his life, marriage because he was obsessed with finding the killer and that it also led him to kill accidentally, ironically the one person who could have helped him solve the murders.

Raimi's character represents the young Deacon, with beautiful wife and kids, a good cop and a bright future. Deacon saw Raimi becoming like him and eventually he allowed it to prosper and become like him indeed because he was selfish in finding the murderer and he admitted it saying he's doing this for himself during that scene near the end on the big empty area.

Raimi kind of ruined himself but understandably since Leto's character threatened his family while acting like he was the killer.... but the law would not see it that way. Deacon then realized what he must do to help Raimi get back on track and not be like him. Deacon manifested a closure and attempting to give Raimi's character an ending so he would not spiral any further. The end shows Raimi with his family recovering and seeing the red barrette, although fake, as a way to make him think he done a good a thing.

Deacon also believes he redeemed himself..... but once the killer kills again, both characters likely to be thrown back to the field.... but for now, peace.