>>28299508I have to assume because committing rape is easier than committing murder and if you don't get them far away before they regain consciousness they'll remember your identity and go to the police.
Eventually all the people involved in that series got arrested anyway though, which is a shame.
If you really want a sex slave the proper way to do it is getting a girl hooked on drugs. Eventually you can entrap her into becoming yours of her own accord when she's no longer capable of taking care of herself. Bonus if you trick her into committing a heinous drug-related crime like trafficking or smuggling, then send an anonymous tip to the police with her info so they'll be looking for her. If she can't even trust the police and can't pass a drug test to get a job she'll have no one to turn to but you. Pimps have been doing this for literally thousands of years and it's a foolproof tactic. 