How long are we going to pretend we don't know that Betty is the killer?
*At the dance her hand was scarred.
*She does everything for everybody and her motive was to avenge her sister.
*Also when Archie tells her he's not good enough she has that look of "if you only knew what I did you'd know I'm the one not good enough for you".
*The way her mom treats her? Pretty clear she had enough pressure to snap.