Alison. We can't discount the obvious. Ali was apparently connected to Bethany, and it's very possible she's been "A" all along. If that's the case, than we have no doubt she would kill to keep her secret. Her expression at the end of the episode, while outside Mona's house, was ambiguous; it could be the look of a killer. That said, the blonde hair didn't look like Ali's cut. Might she have been wearing a wig?
Alison's twin. Nope, sorry, not letting go of this theory just yet. It's possible Ali has an evil twin who is "A." If that's the case, the Bethany-Mrs. D connection could still be in play, but with Bethany's killer and therefore "A" being Mrs. D's other daughter. This could also explain why the hair is more or less Ali's color, but not the right cut.
CeCe. We know another blonde, and this one has been accused of murder before. We know CeCe is supposed to be in France, but who says she stayed there? We don't think she's Big "A,” but might she be doing Big "A’s” bidding? In this case, Big "A" could be Ali — they are very close — or it could be someone else entirely.
Melissa. Hear us out. Mrs. D was apparently having an affair with Bethany's father. So, either Mrs. D had multiple affairs (possible)... or Bethany's father is Mr. Hastings. Could Bethany be a secret member of the Hastings clan, perhaps one even kept a secret from Spencer? And, if so, could Melissa (in a wig) have killed Mona to keep that secret? It seems like a stretch, but we won't rule it out...
One of those twins. Seriously, who were those girls following Mona and teaming up with Ali? Could they be killers?
Wren. What? We're always suspicious of Wren (what is his deal?), he might have reasons for wanting to keep Radley's secrets, and Wren in a wig would be hilarious.
Who do you think killed Mona? Let us know in the comments below!