If you want to kill serially, it's safest to kill prostitutes. They operate in the dark and people rarely care what happens to them. Any attempt to help them is high-risk with low-success. They frequently return to that lifestyle. Many times, pimps get blamed because they do threaten to kill them frequently.
I worked in a trucker motel in college. We had night ladies working there. One of them worked the front desk and she had a large "boyfriend". She discovered I was a virgin and offered me a freebie on my birthday. I knew saying "no" was a bad idea so I started talking about my girlfriend. That saved me.
You do not interface with the working girls unless you want a beating. If you see them coming towards you, you turn around. If you get seen talking to one, both of you get a beating. That's why they make such easy targets.