From this episode this is the major details about the killer:
Gigi Caldwell was seemingly in charge until her death.
Boone Clemens is one of the RDs until his death.
She is the female twin born in the bathtub.
After it being revealed to be the "female twin", these are the characters eliminated from being the killer:
Hester Ulrich - In the promo for next episode is attacked by the Red Devil (unless she hired someone else to attack them)
Chanel Oberlin - ^
Chanel 3 - ^ plus reacted to Boone being alive when she saw him in the park
Libby Putney - ^
Grace Gardner - is Mary Mulligan's child/was born after
Chad Radwell - male and family has been shown on-screen.
Wes Gardner - male and is the father of the twins
Zayday Williams - is a different skin tone to Sophia/Wes/Boone = wouldn't make sense
Denise Hemphill - older
Cathy Munsch - older
Pete Martínez - male
Feather McCarthy - she is currently locked up
These are the characters currently eligble to be the killer:
Melanie Dorkus - the killer always wears the suit, even in front of her family (Boone/Gigi) = could she be the twin and ashamed of her appearance after her accident?
Chanel 4 - all we know is she supposedly died
Any male who has had a sex-change - possibility
After much thought, I don't think the killer is the female twin. I think Ryan planning on a huge surprise since the whole fandom seems to believe that the female twin is the killer.
Female twin - completely eliminates the majority of cast members and it won't be a shock to anyone since there is very little to guess from.
Melanie - would he really make someone who appeared in one scene the killer? No, it will disappoint massively.
Chanel 4 - a randomer wouldn't be good since the audience couldn't make a connection with the character/actor = would be a let down
Male who has had a sex-change - would he really do the whole transgender thing AFTER PLL? Highly doubt it.
So, this is what I think:
I think the killer could be basically anyone who is still on the show. I think the female twin has been kidnapped by the RD (who is posing in the costume infront of the twin's family = Boone/Gigi). I think Boone/Gigi think the other RD is Melanie (who they also think is hiding behind the costume because of her burnt skin) but in reality, it's someone else. This would have a huge shock on the audience. The RD has kidnapped Melanie (which is revealed when the RD brings her out in the finale) so that he/she can pose as her. Then, he/she decided to kill Boone/Gigi which would explain why the killer actually killed them because it doesn't make sense that someone would kill their twin brother and someone who took them in when they were a baby and brought them up.
