Michael Myers killed his sister Judith when he was just six years old, and the reasons behind this act vary depending on the timeline and interpretation of the "Halloween" franchise. The most commonly accepted explanation is that Michael is pure evil—a psychopathic entity with no clear motive or humanity behind his actions. Dr. Loomis, a character in the movies who knew Michael well, describes him as someone with "no reason; no conscience; no understanding... purely and simply evil".
Another past storyline involved a supernatural curse called the Cult of Thorn, which compelled Michael to kill his family as a blood sacrifice to a demon named Thorn. However, this supernatural explanation has been retconned and is no longer part of the main canon.
There is also a director's interpretation from John Carpenter suggesting an underlying Oedipal or incestuous complex as a motive, as Michael witnessed his sister with her boyfriend before killing her, implying an element of vengeance tied to that event. Still, this interpretation is more thematic and not explicitly stated as the definitive reason in the films.
In summary, Michael killed his sister primarily because he embodies pure evil with no rational motive, and any other explanations such as curses or psychological complexes are secondary or non-canonical.
