With a character this simplistic, his brutality is scarier because it seemingly has no logic to it. He's a force devoid of humanity and with unbelievable ability. Michael operates as a force of nature, and this makes him devastating because he faces human characters.
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.