Raven

Finn or asked Noah to kill her father or she killed him herself and Noah took the blame, I think it’s what Issac referring too.

“If you took a little bit responsibility I would have liked you”
left you alone- opposite of what he says next
“But now you have to live in this hell with me”

The reason he came back could be two he is obsessed with her or he wants revenge from Finn because she didn’t say the truth

The flower guy is the serial killer, it makes sense he has tracks in the flowers that he gave her when she went on a date with Issac, also texts her about the flowers

Also Issac seems too know what happened with Noah and his pov is watching from behind usually suspects are put in a small prison kinda somewhere behind the witness stand

so I guessed that the serial killer isn’t actually Noah but the flower guy, Issac is Noah and he just changed his hair and eyes (contacts)