The interdimensional/demon parasite theory
I've liked very much the movie, in fact I think is the best Halloween movie since the first one, although I must admit it leaves hanging all the mysterious mythology surrounding Michael which was presented in Kills.
Anyway, my theory about the Michael mistery that I though after watching Halloween Kills still holds with wha we see in Ends. I explain it:
There is some kind of connection between Haddonfield and some "dimension of evil", like the Black Lodge from Twin Peaks, which can be interpreted like the dark side of the collective unconscious (the Junguian Shadow). When Michael was 6 years old he was possesed by some dark parasite from that dimension, through the window of her sister, and that being feeds himself from fear and pain of human beings. That's the reason he killed her sister in first place. Then he was put in the psychiatric, where he gets catatonic, but the parasite stills feeds from the pain of the other mental patients (like we see at the beginning of Halloween 2018). In 1978, for some reason, what still remains of Michael persona remembers his home and escapes from the asylum. There he stays in his home, maybe the only remain of humanity he still have, but is atracted by Laurie, so he stalks her and her friends. Maybe he didn't pretend to kill her, he just feel some kind os sexual atraction, or maybe she remembers him to his sister and wants to "ask" her about his parents, only human emotional connection he can remember. Anyway, to feed the parasite/demon Michael kills laurie friends, he ends being killes by Loomis but the parasite revives him. H ecomes back to the Myers home where he finally surrenders after killing a cop.
He spends other 40 years in the asylum, feeding from the pain of the crazy patients, until his doctor Dr. Sartain forces him basically to escape again. So all what happens is Halloween 2018 and Halloween Kills is him trying again to come back to his house, to watch for the window, the place where the demon first possesed him. While he goes there he kills a lot of people and manipulates their corpses in macabre ways to create fear, which makes the demon stronger. Finally he gets to the window, then dissapear. That's the most confuse aprt for me, I don't have clear what happened there, nether what Karen saw from the window in the ending of Kills.
Anyway, Michael dissapears and the Myers house is demolished. He is forces to hide in the sewers, where during the next years he catches people and kill them, feeding from their fear. Until he mets Corey, and what happens then? I understood the parasite can see through people minds, and when it feels all the pain Corey is suffering it enters him, like a demonic possession, but not leaving Michael compeltely. So the parasite (the "evil" as they call it in the movie) is now divided between Michael and Corey, which I guess makes it weaker. And finally we have the climax, where both Corey and Michael, the demon recipients, are destroyed by Laurie. I understand Laurie has been all her life obsessed with Michael, but when finally confronts him she has overcome her fear, so Michael can't get power from her, and that allows her to finally kill his body, and presumably also the parasite inside, which, along with Corey death, destroys it forever.
Also all the town makes that ceremony to get rid of the "Evil" forever, curing themselves in a collective way.
Obviously this theory is just implied, but I think David Gordon Green leaves enough clues so we deduce it... Because it's obvious Michael wasn't just a crazy man.