Monsters: The Lyle and Erik Menendez Story tells the sequence of events leading up to Jos and Kitty Menendez’s deaths and the fallout afterward, which involved charging and convicting Lyle and Erik Menendez for their parents’ murders. Monster season 2 follows the Menendez brothers’ story after the success of the show’s first season, Monster: The Jeffrey Dahmer Story. On August 20, 1989, Lyle and Erik Menendez surprised their parents with shotguns and killed them. Although the police did not initially suspect Lyle and Erik Menendez, their psychologist’s mistress turned in the tapes of Lyle and Erik confessing to killing their parents.

https://screenrant.com/why-erik-and-lyle-menendez-killed-their-parents/