Okay- so after watching the finale (if you didn’t see it coming then you don’t have the brain cells to watch it) I have this theory about the 3rd ORIGINAL killer.
All we heard about Brandon James is that he was kind… Loved Maggie/Daisy and didn’t like the way that Kevin treated her. Then BAM suddenly Brandon was a serial killer and killed Kevin’s friends ( a little bit suspicious that HE was the one who got away).
Now I don’t think Brandon killed Kevin’s friends, nor did he attack Kevin. I think that Kevin set him up and framed him for the murders- and Maggie and Acosta were both present and saw that Brandon was innocent, thus hiding the knife.
The final scene with Maggie features her looking at a note, attached by a KNIFE (the one that Acosta and Maggie buried?). Not sure about the ’ stay away from her’ concept but maybe Brandon wants revenge on Maggie, Acosta and Kevin because they covered up the fact that he was framed and didn’t say anything which besmirched his name too. What better way to hurt Kevin (and mainly Maggie) by targeting their daughter and killing her? I think it’s Brandon James OR somehow his brother Troy is linked to it. After all, when the phone rang for Kieran he did say ‘MY mask’ and we know that Troy made the mask for Brandon.