The real horror is that Michael could be anyone. He could be anyone's neighbor. He could be in anyone's household. Michael is the shape of a human but is a vessel of evil, which is why the simplicity of his look and actions are so important.
The reason Michael Myers is so scary is because he can tap into our fear of the unknown. There's a very uncanny valley quality about him. He's in the shape of a man, but we can tell that something is off about him.