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: 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: 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.