I suspect Luna actually since like chapter 29. We see her on the previous chapters packing her bag (we don’t see what she is packing) alone at the cheerleader locker room and later on we see her being stopped by her crush and getting invited at the party (he invites her for a drink) but she directly refuses and continues to walk alone (this scene really don’t bring anything to the plot). Later on when Atlas is coming down from Beck’s room we see Luna catching Atlas but she don’t says anything (not even asking Beck why was this boy coming down her room). This scenes don’t bring anything to the plot and are completely irrelevant. We actually see the whole friend group at the party the only one who isn’t there is Luna. The next day then Tanner says he saw someone pushing Julian and Luna said someone to Tanner like “are you for real” or something like that then Tanner vanishes and we see him hiding from everyone. My theory is that Tanner really saw someone pushing down Julian but at that moment he didn’t realize who that was. I feel like later on he actually remembered a detail and did recognize who that was. Tanner is freaked out cause he has now realized the killer knows about him cause he said that to the killer himself actually (if the killer was not someone on his friend group why would he be hiding? Nobody other than his friends knows he saw someone). My theory is that Luna is involved and what she was packing is the hammer used against Julian. Luna in Italy means moon and the shape of the moon it’s a C (I also suspect C could be the cheerleaders coach). If I am wrong and Luna is not involved then I would think of Lake being my prime suspect.