In the end, it's revealed the Liars' school principal, Marshall Clanton, was A all along. While the revelation comes as a shock to viewers, it's not entirely out of left field. Principal Clanton was seemingly waiting in the wings throughout the entire season. For example, it explains why the masked killer could roam around the school, evading notice. It also explains why A was in the know about everything the Liars were discussing and planning throughout the season.
So, what was Clanton's motive? It turns out Clanton is Angela Waters's father. He grew up with Rose Waters in Millwood and they fell in love while in the school choir together. When Rose became pregnant, Clanton's parents didn't approve, so they paid Rose off, and she disappeared. Clanton went on to become the principal of Millwood High. Years later, Rose returned to the town because she wanted Angela to go to Millwood High. Clanton agreed, as long as Rose kept his paternity a secret. From then on, Clanton had a front-row seat to the mothers' mistreatment of Angela and became A to exact revenge on them after her suicide.
But Clanton had some help.
