I'm wondering if anything this is going to be a direct sequel to the original Halloween and it retcons the veents of the sequel. John Carpenter repeatedly puts down the brother/sister reveal as a consequence of drinking a 6 pack of budweiser as he couldn't think of another reason for Michael to be stalking Laurie. Blank character and absence of motivation would make Michael more scary as that makes him a representation of evil!
Basically, after Loomis shoots Michael 6 times off the Doyle house balcony the Shape simply vanishes in the night and isn't heard from for 40 years.
I think it's not the lack of motivation, but it's more that his triggers are
so small that it makes them scary, as seemingly everyday occurrences could set him off.
For his first kill, his sister was fucking around (literally) when she was supposed to be taking care of him. That set him off and apparently turned him into the monster. His parents stopped him from leaving the crime scene and he got apprehended, and he had no reaction to that, because getting caught is not a trigger, so it doesn't matter to him.
Then after being imprisoned for more than a decade, he randomly decided that it was Halloween (the anniversary of his first kill) so he wanted to "go home" (which includes stuff like visiting and then stealing his sister's tombstone), so he broke out of a mental ward like it was nothing. He killed a guy for his clothes, and killed/ate a dog. That was presumably not personal, but maybe the guy cut him off in traffic.
Then Laurie's infraction was that she stepped onto the porch of Michael's home, when the neighborhood kids were afraid to set one foot on his property. It probably also helped that Laurie slightly resembled his sister. So Michael starts stalking her, studying his prey.
During the stalking, Laurie meets up with two more girls, and one of them yells at Michael. Michael slams on the brakes of the car and I'm pretty sure we're meant to believe that Michael was moments away from road-raging and flipping out, running out of the car to get all stabby on the three of them in broad daylight, but he managed to calm himself down, added murdering the two new girls to his mental to-do list, and resumed his stalking pattern.
Michael's "reason" for stalking Laurie in Halloween 2 should be as simple as "she's not dead yet". He tried to kill her but she's still there. He's patient and methodical and determined, and if Laurie got away from him it would seem likely that nothing else in the world would matter to him (beyond being an obstacle), he would still be after her, unless something else created a temporary distraction by triggering him.
