Question: who actually killed mona in pretty little liars?
Answer: She is believed by the Liars to be Alison DiLaurentis. This person then kept Mona as a personal Alison doll in her dollhouse. In "Welcome to the Dollhouse," it's revealed Mona is indeed alive, and was not killed but kidnapped by "A". In "Game Over, Charles," it is revealed that CeCe Drake was the assailant.