I feel like there's two different sets of killers, Chanel & Boone together, Chanel getting friendly with Boone at the coffee shop (Chanel would be the one in the costume who visited Boone and then gets him out of the morgue) and then Munsch & Pete, since Pete also has the devil costume as the mascot and gave a weird look to Grace once she ran away, Pete would also be the baby and Munsch would have adopted him or something after covering up the death of the mother.
I don't have any ideas why they would be killing in the first place though. I know this post is also a bit of a reach.