Devil_666
Super Freak
But I still say he wasn't mental. There is nothing to suggest he was disabled before he killed his sister. I believe it was meant to be taken as though nobody could have seen it coming.
Just to clarify, I'm not suggesting that Michael Myers was/is mentally retarded (I think the P.C. term is "Intellectual disability"). I'm saying he was/is mentally ill just like Norman Bates, etc.
And mental illness is considered a form of disability.
So as far as Michael Myers having the mind of a child, that's simply because he was institutionalized at a young age. Somehow I doubt he received the same education in a mental facility that a normal child would have outside. So even though he eventually became an adult and looks like a grown man, he still has the mental capacity of a child. But again, he doesn't have an intellectual disability.