Since "Pretty Little Liars" began, almost everyone has been suspected of murdering Alison DiLaurentis (Sasha Pieterse). From Spencer Hastings (Troian Bellisario) to Ali's brother, Jason (Drew Van Acker), there is barely a single person who has escaped speculation. New revelations cropped up during the Halloween special, showing that Jenna (Tammin Sursok) believed Garrett (Yani Gellman) killed Ali, but he didn't. The last person who saw Ali alive was, in fact, Aria's dad, Byron (Chad Lowe). So, could he have been the killer? Let's take a look at possible motives.
Ali knew about his affairThe most obvious motive for Byron killing Ali is that she knew about his affair with Meredith (Amanda Schull). Not only did Ali know about it, she caught him in the act and later helped Aria (Lucy Hale) frame Meredith for trashing his office. We know Ali had a weird obsession with revealing truths, so perhaps Byron needed to be rid of her to keep his secret safe.
Ali may have blackmailed himAs well as a desire to reveal everyone's secrets, Ali was also severely twisted. She made a lot of threats to a lot of people, so it wouldn't be too much of a surprise to find out she tried to blackmail Byron. If this was the case, Byron wouldn't take it lightly. Proof of Byron's short temper has been seen many times throughout the show's run, and Ali's snide comments could have been enough to make him snap.
He was trying to protect AriaAlthough the other pretty little liars considered Ali to be their best friend, since her death, they have all realized she treated them like puppets for her own amusement. It wouldn't have taken a genius for an adult to see it, though. Byron has always been fiercely protective of his daughter, and he would never stand for anyone treating her badly. Alone, this probably wouldn't qualify as a reason for Byron to turn into a killer, but with her also holding the knowledge of his affair, it might have been just another reason for him to need her out of the way.
He was having an affair with AliThis is a popular theory among the hardcore "Pretty Little Liars" fans. You see, there is actually just enough evidence to back this up. Alison had a thing for older men, and Byron was obviously not against having flings with people who were much, much younger than himself. She appeared to be livid when she discovered Byron's affair with Meredith, but why was it a big deal to her? Her level of compassion for her friends was mediocre at best. It stands to reason that she was also having a thing with Byron, which could have been her motivation for wanting to ruin his life. There are comparisons to Ali wanting Aria to tell her mother about Meredith, and the way Ali behaved when she saw Spencer kissing Ian (Ryan Merriman), who she blatantly had a crush on. She didn't like anyone seeming "better" than her, and it became her mission to take them down.
If Ali really wanted to make Byron's life a misery, all she had to do was threaten to tell everyone he'd been with an underage girl, and his life would be over. If that secret got out, not only would it have ended his marriage, it would also have ended his career.