Pretty Little Liars may have finally revealed one of its season-long secrets in "Farewell My Lovely" when it flashed back to Mona and Charlotte in the church tower on the night of Charlotte's death. But can that flashback be believed, or is it all just one more red herring? Did Mona kill Charlotte on Pretty Little Liars?
Mona was responsible for Charlotte's death, but Pretty Little Liars played coy on actually showing the act itself at first. It cut between the past and the present as it revealed what finally happened that night. After suspicions were raised that Mona might be A.D., she retreated to the church tower done up in her old school braids and glasses, seemingly regressed to an earlier time. That was where Hanna found her, at which point Mona told her just what went down the night Charlotte died.
Fresh off her release from Radley, Charlotte had been ready to start the A game all over again, but Mona wasn't willing to let that happen. She didn't want Charlotte to hurt her friends anymore, so she threatened Charlotte by pushing her to the edge of the open window and telling her to jump. But Mona couldn't go through with it – at least, not until Charlotte did a little pushing of her own.
Charlotte knew Mona wouldn't be able to kill her and she couldn't resist taunting her about it, which proved to be a big mistake. Charlotte said that Mona was doing all of this for a group of girls who didn't even care for her half as much as she cared for them, and who would never accept her. That enraged Mona enough to incite a physical fight between the two of them. In the ensuing melee, Charlotte knocked her head against a pipe in the wall. That was what killed her. Presumably Mona then pushed her out of the tower to cover it up.
As far as reveals go, this one was on the anticlimactic side. Despite so many characters having more than enough of a motive to take Charlotte out after the years of torment she inflicted on them, her death was a total accident. The episode ended with Mona being taken to the hospital after having some kind of break with reality due to the trauma of reliving her memories of the murder.
Discovering Charlotte's killer ended the game. A.D.'s mission, such as it were, was accomplished. The only question left is the biggest one of all: who has been behind A.D.'s attacks all season long? That answer will have to wait for the series finale.