@Tnecniw It’s a more extreme form of S&M, same as a lot of other potentially “weird” fetishes like Giantess, Vore, etc.
If you track fetishes down to their base psychological level, it’s much easier to understand them. Usually it boils down to sexual orientation, sadomasochism, an attraction to “defilement” or “taboo”, and/or a combination thereof.
If she's a masochists who enjoys feeling pain, Then wouldn't the ultimate form of torture to her be a simple non-violet dinner date followed by hand holding sex?