The killer called Amanda. He told her "Sexually explicit things" he did to Melissa, and he told "Sexually Explicit things" he'll do to Amanda. Isn't that a rapist?
What is the significance between calling him a sadist and calling him a rapist, aside from the psycological aspect?
When you say "sadist" it points to a member of a BDSM club, or purchaser of torture toys etc. (Do you think he has those? I don't)
When you say "rapist" it points to a registered sex offender. Which I believe I found a quite viable suspect in there. I wrote a lot about recidivism. Please read it.
Other than that, calling him a sadist is pointless. Of course he must be some kind of sadist, he kills people.