Well since you are looking for an idea, I could provide with something different for your story that maybe you might like this approach?
[just to be clear : there will be no sexual stuff, sorry but I'm not familiar with those.]

1. Most people are picking the helpless or naïve one. I would suggest to say having a serial killer who targeted the level headed, smart one that pose a fighting chance and thinking that they are invulnerable. Because it would be more fun for the serial killer to break them.

2. Fear is a one strong tool to throw people off balance, instead of focusing on physical harms. The serial killer would be playing with the fear on the preys they has eyes on. subtly playing on their paranoia while building up on their own found weakness, toying with them, psyching them out, while making sure that only the prey will see those and nobody else.
But in the same time also giving them a room for break so they can fell into the false sense of security. When it's time to deliver a killing blow, the serial killer would drop all subtlety and throw everything at the prey at once, sending them over the edge. People often do terrible decision in panic and after breaking them they will eventually get themselves killed trying to get away [a slippery surface around a long fall might help ensuring the job will be done sooner]

3. There would be no need for it, Since the trophy would be all over the news and internet for everyone to see "random stronk woman got themselves killed in the most humiliating way possible."

4. If step 2 were done with no trace, People would established it off as an urban legends. A story to scare the weak, and potentially creating more prey for those who dare seeking out.