They are definitely trying to lead people into betting on either
Shauna
or
Taissa
but I think in the end it will be neither and instead they will show it was someone most wouldn't have expected. Either that or they'll blue ball us all and reveal she really did just
fall down the stairs
in an anticlimax a la Travis's death reveal.
Cast In The Name Of God…Ye Not Guilty

not really interested in that present day plotline, but i'll play:
plot twist: it was Taissa or rather her other evil dirt-munching personality.
so there, i said it.
the sound of your racing heart

well, I'm not entirely sure but some people have suggested that
HE is actually Hannah's SON alex, if that was the case then he could've helped the yellowjackets with the adam martin situation to prevent cops getting in his way for his own plans with them…I thought that could be, except it seems the character Alex is already credited to a young actress…side note Hilary Swank is not Alex either afterall, and they confirmed Melissa is dead so I wonder who she is playing
aside from that it could just be
That he is an insane murderer, I mean he isn't normal, he knows the yellowjackets brutally murdered Adam and is completely fine with it and helps them cover it up with another murder. He is fine with the cannibalism too and even mentions his favorite musical is sweeney todd, the guy is nuts
This show really likes to toy with expectations and throw curve balls so I just don't think it will be anyone that they've obviously set up already.
Cast In The Name Of God…Ye Not Guilty

They are definitely trying to misdirect us with
the DNA evidence under her nails
, but what would the motivation be for
Walter to kill her
?