These words were taken directly from a scene in the original 1980 film in which Pamela Voorhees, played by the late Betsy Palmer, says Kill her, mommy!  in Jason's voice. Manfredini said the scene stuck out as a creepy shot  and also conveyed the important idea that she was hearing voices at all times.

According to IMDb, composer Harry Manfredini's film score is meant to sound like young Jason's voice saying "kill, kill, kill; mom, mom, mom," inspiring her to go on a killing spree. Manfredini created the effect by speaking the syllables "ki" and "ma" into a microphone running through a delay effect.