Love this movie but the killers motive was disappointing

To tell you the truth, I cannot even remember the basic plotline of Melissa Sue Anderson's character though I have seen the film multiple times over the years--something about a sensitive brain operation. But the movie became a triumph of marketing. As an eleven year old at the time, the posters and ads promised "Six of the most bizarre murders you will ever see" and that was enough to make me want to watch it. That shish-kabob ad promised trashy, gory thrills the movie didn't exactly live up to. At a running time of almost 2 hours--an eternity for a horror film--the film became plot-heavy when most people who paid their tickets to see it were looking for the gore scenes which were few and far between with long dull stretches in the middle and too much backstory and exposition. It did not exactly catapult Melissa Sue Anderson into starring roles after her Emmy-nominated work on Little House on the Prairie. The movie needed a trashier sensibility to succeed as a memorable slasher movie.

Haven't seen this years. Love this movie.
To do what she did I really don't think you need a good reason.