It's hard to ignore the fact that someone only targeting Sherrill had all evening and half the night to attack her.
From the evidence & timeline, though, the killer didn't appear until shortly after the girls arrived home from partying. If someone was stalking Sherrill that night, someone who had a grudge or wanted to have sex with her, why would they wait until the other two girls arrived home? It's not impossible, but the timing of the attack just after the girls came home doesn't seem like coincidence.