Michael Myers, the shape, the boogieman, the mystery of horror films. This masked serial killer is an icon for a reason. He keeps frightening the audience with his lack of emotion and unpredictability. Michael is described as pure evil by Dr Loomis and many would agree: his only reason for living is to kill. But what is the motive behind the killing? The films don't answer this question in Halloween (1978) and its sequel Halloween 2018 which both happen in the original timeline. Laurie Strode isn't Michael's sister and there is no curse involved, so what else drives him?
Michael is showed to be 6-year-old at the beginning of Halloween. He kills his older sister Judith after this had sex with her boyfriend meanwhile the parents are away. She was brutally stabbed to death in her room. Now that sounds unarguably evil and deranged. Can any six years old boy do anything like this in reality? Yes but it's extremely rare. Psychopaths are born, not made. Scientists have figured out that psychopathic traits can be recognised in early childhood through testing out fear response for example. Amygdala is the main producer of this response: if it doesn't work properly, every emotion connected to fear turns shallow. This does include affective (emotional and bodily feeling) empathy which differs the majority from psychopaths. Michael does not understand the fear because he doesn't have it. Thus why killing for him does not cause panic or worry for consequences and seeing others in pain does not create a similar reaction. What it may create is curiosity instead. Psychopaths are also prone to boredom more often since their emotional life is lacking. Michael may find killing as an activity that brings him excitement even though he might not understand this himself.
Psychopathic traits explain a lot in killers but nurture does play a part in everyone's development. Michael's origin story doesn't tell much about his family. His parents were elsewhere and seemingly left Judith to babysit young Michael. I have a suspicion that Michael didn't get attached to his family which would explain why killing his sister seemed so easy. Neglect takes more than one form: it can mean emotional neglect as well. In this case, the caretaker takes care of the child's necessary needs but leaves out affection and warmth. Since Michael is psychopathic, children like him need more guidance to learn the right from wrong. When it's not provided, the child's attachment to their caretaker is avoidant. Michael may have acted normally on the outside, but wasn't feeling any love inside for the people around him. He sees humans as targets instead of living beings. The meaning of sister is not self-evident for him because of the way his brain works. And this paradigm gets even worse after Michael is sent to a mental health facility where he is kept locked up from the outside world for many years. You can only guess what this does to a growing child.
When it comes to Michael's intelligence, I say his analytical thinking is average or higher. This would explain his intentional mutism (Dr Loomis and Dr Sartain tell that he can speak but chooses not to) and his fast learning skills (somehow he knows how to drive a car). It's common among psychopaths. What Michael is missing is other sectors like emotional and social intelligence. It's nearly impossible for this kind of person to understand abstract ideas related to emotions and social behaviour. They can only get ideas that are represented concretely, for instance, how to use a physical object correctly - or what things make people scared. Michael may understand what fear, anger, happiness and sadness look like on someone's face because it's physically visible, but might not get the reason why his behaviour is considered creepy. He just hears what others think of him and takes advantage of the information. Perhaps him wearing the mask is linked to this: he knows that hiding his identity gets him the wanted reaction out of his prey. Who knows if Michael is smiling smugly under the mask every time he manages to cause panic by just standing still. And from here we get to the main motive.
Michael kills people because he enjoys it. I don't think there's any hatred towards the society since he has never been a part of it. It's a cat-and-mouse game to him where he's an excellent hunter. It's like it was meant to be his only purpose in life: Michael acts catatonic whenever he's not out killing is a sign of this. Dr Loomis saw through him and thus why gave up on Michael after eight years and focused on keeping him locked up from that point on. He knew Michael would end up murdering random people and he did in both Halloween films. Now, why did he choose Laurie? She makes a good prey by being aware of her surroundings, unlike her friends. Michael senses this, knowing he is already scaring her which makes the stalking more fun. If only Laurie wouldn't have cared, things could have gone differently for her. The problem is whenever Michael sets his eyes on someone, that person will be his next target no matter what. That's why he is to be feared.