Michael Myers was a boy who, on the surface, was just like any other boy. Voices whispered to him in the dark, though, and one Halloween night he did as they bid him to. He put on a clown costume, and took a butcher knife to his older sister. She died, and Michael waited for his parents, and the police, to come retrieve him. Years passed in the ward, and it was long years where Michael destroyed the minds of those who tried to help him. Those who gazed into his abyss, and felt the empty hunger that waited behind his eyes. Eyes behind which lurked pure, unadulterated evil.
For those who'd like to add this iconic slasher to your Pathfinder RPG game, this guide should help you get started. For guides to other slashers, like the infamous Jason Voorhees, as well as characters like the Avengers, the cast of Game of Thrones, and others, the Character Conversions page at Improved Initiative has you covered.
Race, Attributes, and Traits
Despite the terror Myers represents, and the assertions of Dr. Loomis that he isn't human, that's the best starting race for Michael. Though the human trait Dim-Dweller (which gives you darkvision and a +2 bonus on Stealth and Intimidate checks while in areas of dim light or darkness in exchange for your 1 bonus skill point every level) would make sense for how Michael is able to do what he does in the films. Also, despite spending most of his life in a sanitarium, Michael seems to have no shortage of Strength, Constitution, or Dexterity. Though he doesn't speak, and we rarely see his face, there is a presence that comes with him. Along with, that is, a will to keep going no matter what happens to him.
As to Michael's traits, perhaps the most appropriate is Killer (deal an additional amount of damage on a critical hit equal to your weapon's critical multiplier). Other traits like Cold Heart (+2 trait bonus on saves against effects with the emotion descriptor), or Criminal (gain a +1 trait bonus on Disable Device, Intimidate, or Sleight of Hand, and that skill is always a class skill for you) would also be appropriate.
In addition to traits, though, you might want to consider some drawbacks for Michael. Something like Burned (-1 penalty on saves against fire, and a -1 penalty to attacks, saves, and skill checks when you are adjacent to fire) or Infamous (you and your allies take a -4 penalty on Diplomacy checks interacting with law-abiding citizens) might fit quite nicely.
The Man Behind The Mask
Class and Skills
Michael Myers is an anomaly. In an age of scientific enlightenment, psychologists cannot explain his nature. There is no understanding what Michael is, until you learn that science is a candle whose illumination does not extend far into the shadows. There is no understanding the nature of evil, especially when that evil is determined to wipe out its entire family line... along with anyone who gets in its way.
Michael's dogged determination to hunt down every member of his line, and his determination to wreck bloody ruin on anyone who gets in his way, makes him an excellent candidate for the Slayer. More than that, though, the Family Hunter archetype (Horror Adventures 66) ideally suits his purpose. While most of Michael's victims aren't actually related to him, that doesn't make him less capable of putting a serious hurt on anyone who would try to protect his sister, his niece, or anyone else he comes after.
When it comes to Michael's slayer talents, there are a few, stand-out choices. Eerie Disappearance (as a full-round action, move up to your speed and into cover. If an observer doesn't succeed at a Perception check, they have no idea where you went) is great for a slasher, but so is Slowing Strike (enemy damaged by sneak attack has its speed halved). And, of course, Defensive Roll (when a rogue would be dropped below 0 hit points by an attack, he can make a Reflex save to roll with the blow for half damage) would explain how it is Michael can take so much punishment, and keep getting back up.
Additionally, if you want more damage with Michael's signature weapon, you might want to dip into the Knife Master rogue archetype (Ultimate Combat 72). This changes your sneak attack dice to d8s with knives, but to d4s with other weapons.
For his skills, you'll want to invest in Perception, Intimidate, Stealth, Survival, Acrobatics, and Knowledge (local).
Though Michael's primary skill set is Stealth (and it can be increased with feats like Skill Focus [Stealth] or Stealthy), it's the trail of bloody bodies he's left behind, as well as his own refusal to die, that people remember. While there is a twinge of supernatural vitality in him, much of Myers' deeds can be executed with good, old-fashioned feats.
The two feats you'll want right off the bat are Endurance and Die Hard (Core Rulebook 122). These feats allow Michael to keep going long past the limits of normal, human endurance. Especially important considering the sheer amount of punishment he soaks up. Other feats like Fast Healer (Advanced Player's Guide 160) and Godless Healing (Inner Sea World Guide 287) will allow you to make the most of what rest and healing you receive.
When it comes to combat, you have a lot of options. Meyers' signature weapon is a butcher knife, but that measly 1d4 isn't where most of the damage comes from. Feats like Power Attack (Core Rulebook 131) add a lot of damage, especially when paired with feats like Furious Focus (Advanced Player's Guide 161) and Bloody Assault (Advanced Player's Guide 150) which allow you to ignore the negative from Power Attack, and deal bleed damage respectively. And for those times when Michael fights with something that isn't his trusty blade, Catch Off-Guard (Core Rulebook 119) is a good way to make sure you get all your sneak attack dice. And, if you have the leftover slots, Improved Unarmed Strike and Improved Grapple (Core Rulebook 127 and 128 respectively) round out the options.
He Isn't Human!
Myers is unique among slashers, in that he never dies. Despite taking a full cylinder worth of bullets from a .357 Magnum, being set on fire, put into a coma, beaten, cut, and brutalized, Myers gets up every time, and keeps on coming. It's not that he's been raised from the dead... it's that he was never killed in the first place!
One way to represent this ability (especially since later films suggested Michael is the unwilling recipient of the dark forces of Halloween itself) is through the Undying Hatred manifestation of the Accursed corruption (Horror Adventures 17). When you would normally be killed by hit point damage, you fall to -1 hit points, and stabilize. Unless the observer looks at you closely, and succeeds at a DC 25 Heal check, they think you're dead. After 1 minute, you gain 1 hit point, and become conscious. The stain, of course, is that whenever a foe is helpless and within 5 feet of your reach, you must make a Will save or perform a coupe de grace. Something that Myers seems to have very little problem with, since stopping to kill another victim might be all it takes to give his real target time to escape.