It was originally called It's A Wonderful Death, and he was going to be named Michael Bailey.
The story of Michael Myers' fun-sized murder spree was soon followed by Halloween II. But as we've pointed out, it was actually more of a sequel to the TV version of Halloween. It introduced the twist that Myers' target, babysitter Laurie Strode, was in fact his long-lost sister. So to make sense of an idea he came up with out of desperation and drunkenness, John Carpenter went back and George Lucas-ed the original, adding new scenes for the television broadcast.
So you can see already, our homemade map of the series is starting to look as even as the Hamburglar's cardiogram:
Complicating things further, the next movie, Halloween III: Season Of The Witch, abandoned the story of Michael Myers altogether, with the goal of turning Halloween into an anthology series. So instead of a murderous masked man, it concerned a deadbeat dad battling an evil businessman who's using giant computers and a hunk of Stonehenge to magically kill children with an evil TV commercial. Seriously.