Trivia: The "Bride in Black" is played by a man wearing women's clothing. This was done so that something would feel "off" about the image of the ghost. At the time, the character was intended to be female. However, when the creators later reused the character for the sequel and added in a backstory, they worked the fact that the character was played by a male as part of his backstory - it actually is a man who masqueraded as a woman as a disguise to lure in victims.
Trivia: The main demon was actually played by the film's composer, Joseph Bishara. The child actors involved in the film found him incredibly frightening while in the full demon makeup and prosthetics. Director James Wan tried introducing the children to Bishara so they wouldn't be scared anymore, but even after meeting him and realizing he was a nice, quiet, friendly man... every time he would put the makeup on and start acting, they would become frightened and start crying.
Trivia: Before the "dancing boy" ghost is first properly seen, eagle-eyed viewers might notice that as Renee is walking through the house, he is briefly seen for about a second and a half, facing the wall in the laundry room. The camera quickly passes over him without calling attention to him. James Wan added the moment as a very subtle hint at what was to come - many viewers didn't necessarily notice him the first time they watched the film, but could subtly tell that something was there that shouldn't have been. Wan later replicated this in a similar scene in the second film.
Trivia: Both this film and the original "Saw" were directed by James Wan and written by Leigh Whannell. In addition, both films share some other interesting peculiarities. Both were micro-budget films with a production budget of just over $1 million. Writer Whannell plays a major character in both films. Both were made without a guaranteed theatrical release but earned positive buzz in the festival circuit that got them major releases. And both were massive hits at the box office and despite being intended to be solo-films, both ended up becoming franchises.
Trivia: Ethan Hawke was considered for the lead but ended up having to turn it down. Though the story ends well, as he was able to star in the similarly-themed "Sinister", which was made by the same production company and producers, and came out about one year after "Insidious."
Trivia: The "Long Haired Fiend" (the ghost with long hair and the long, black coat) is portrayed by J. LaRose. Like director James Wan and writer Leigh Whannell, LaRose is also an alumna of the "Saw" movie series. LaRose had a small role in "Saw III" as a victim of the "Jigsaw Killer."
Trivia: Josh's mother's name is "Lorraine." This is a reference to real-world paranormal investigator Lorraine Warren. Director James Wan later went on to direct "The Conjuring," a film loosely based on the case-files of Lorraine Warren and her husband Ed.
Trivia: When Renee finds a red handprint on Dalton's bedsheet, the implication is that it is blood. However, the demon in the film was originally going to be shown applying lipstick to his face to give it its fiery look (behind the scenes material show the filming of this deleted scene), and it was going to be revealed that the handprint was actually red lipstick, showing how close he was to taking hold of Dalton.
Answer: It's implied that the girl went crazy and killed her family, but it's not really elaborated on. They're just some of the many spirits to be trapped in and haunt the Further, likely due to the violent nature of their deaths.