Michael Myers is a virtual catalog of mental illness. This page is a list of them.
Michael has a disorder called catatonia. Michael Myers is sometimes disabled from moving whenever he either sits or stands. This makes sense because it explains why Michael walks after his victims rather than runs. He exhibits stupor also which is an inherited disorder. Dr. Loomis asserts that Michael was faking his catatonia to draw them away from his plans of escape and murdering his family. This is quite likely, and if this is true, Michael is more psychotic than catatonic.
In Michael Myers' case, he experiences an extreme loss of motor skills or he can even experience hyperactivity (in his case murdering people that he sees). When he was in Smith's Grove Sanitarium, he expressed rigid poses for hours and ignore any external stimuli. Michael also seems to be psychotic, engaging in zoosadism and mass murder, however he may also be sociopathic, which means a total lack of emotion.
It may be proven that Michael Myers may have schizophrenia because of the reason most people that have schizophrenia experience catatonia also. The novelization of the original film gives more detail into Michael's schizophrenia, explaining that he saw visions of a disabled Celtic child named Enda who murdered his crush after she refused to acknowledge him. He also claims that he heard "voices" that told him to hate people.