Originally Posted by
Misawa
It was probably intended to originally to just be one film, but monetary success = sequels.
Yes, I believe the story was partly based on the Manson "Family" murders. Charles Manson is who you're thinking of. They invaded the home of Hollywood actress Sharon Tate and murdered her and her partner, and cut out her unborn child. Manson never actually murdered any of them, but waited outside of the house and orchestrated the whole killing. Sickening stuff.
Indeed, sometimes reality in films is boring, and otherworldly elements are far more enjoyable and fresh, but in the horror genre, both realism and unrealism can work to great effect, for example, Jason Voorhees and Michael Myers are seemingly immortal, no matter what happens to them physically, that is totally fantastical, but is what makes those characters who they are. Realism or the cold truth, such as in The Strangers and Funny Games, scares the audience because it could actually happen to them.