Why Did Michael Myers Kill His Sister? 'Halloween,' Explained

Michael Myers has been terrorizing the fictional town of Haddonfield, Illinois for more than four decades, cementing his place in horror history as the slasher villain with one of the highest kill counts, totaling more than 160 by the end of 2022's Halloween Ends. The Shape's reign of terror began way back on Halloween night in 1963 when Michael, then just 6 years old, violently killed his teenage sister Judith Myers. But why did Michael Myers kill his sister in the original Halloween?

While the 13-film Halloween franchise has included several different continuities and storylines, one thing has remained the same: on October 31, 1963, 6-year-old Michael, dressed in a clown costume, stabbed his sister to death with a kitchen knife as she sat in front of the vanity in her room. Judith's murder, which resulted in Michael's years-long stay at Smith's Grove Sanitarium, marked the first death in what would become a bloody, decades-long rampage across Haddonfield, but the reasoning for Michael's bloodlust at such a young age largely depends on which continuity you follow.

The most popular theory, and the one that has prevailed the most throughout the various Halloween films, is that Michael, simply put, is the embodiment of pure evil. Michael had no real reason to kill Judith, portrayed by Sandy Johnson in the 1978 horror flick, or any of his other victims, but, as Dr. Loomis put in the original 1978 film, "there's an instinctive force within him."

"I met him, 15 years ago; I was told there was nothing left; no reason, no conscience, no understanding in even the most rudimentary sense of life or death, of good or evil, right or wrong. I met this... 6-year-old child with this blank, pale, emotionless face, and... the blackest eyes – the Devil's eyes," Loomis said in one of the most recognizable quotes from the franchise. "I spent eight years trying to reach him, and then another seven trying to keep him locked up, because I realized that what was living behind that boy's eyes was purely and simply... evil."

While Michael has long been touted as the embodiment of evil, Halloween: The Curse of Michael Myers, the sixth installment in the franchise released in 1995, posed another explanation: the Cult of Thorn curse. Set six years after Michael's last attack on Haddonfield in 1989's Halloween 5: The Revenge of Michael Myers, The Curse of Michael Myers introduces a cult of druids who, in order to preserve their society, place the Curse of Thorn on a child who is then compelled to kill their entire family on Halloween night. When he was just a child, Michael had the curse placed upon him, and under the influence of Thorn, killed his sister.

The Cult of Thorn proved to be among the more controversial entries into the Halloween franchise, with Halloween 6 ranking as the worst-rated Halloween movie on Rotten Tomatoes in terms of critics score, which sits at a measly 8%. The critics consensus on the site blasts the movie for trading "the simple, brutal effectiveness of the original for convoluted mysticism, with disastrously dull results. As a result, none of the films after the 1995 movie, including the most recent David Gordon Green trilogy, followed the Cult of Thorn theory, instead falling back on Dr. Loomis' explanation that Michael is just evil and kills with no clear purpose.