Life is a (personal) journey.
The knee jerk reaction is tell people what they should be looking for in a mate and how unhealthy it is to be submissive in a relationship. However there is nothing anyone can say that will make another feel differently about what gives them pleasure or (their) preferences.
You can't tell someone what makes (them) feel good or happy!
There is no such thing as an "universal deal breaker". Whatever you or I could come up with there are people living under those exact conditions who have NO plans of ever leaving!
I would not encourage a person to stay. I could only tell them why that would never work for me. And then I would ask them why they found it "appealing". Even those who are into BDSM aren't open to being killed. They usually want some type of "safe word" to end the punishment! Maybe this person is depressed or suicidal.