The ending was changed in the wake of the fallout of multiple sexual assault allegations being made against male members of Hollywood. The original ending was left open ended to leave room for a sequel. Once convinced that the motive was strong enough for the killings to have been repeated, the ending as seen was included in the released version of the movie.
One of few films featuring twin female serial killers, the twins are played by real life twin female identical adult twins as opposed to one actress playing two roles. Some of the audio and video was modified to conceal the ending, as can be seen in the use of "night shot" in the ending and in sequences before the reveal.