Detachment goes hand-in-hand with stoicism. How are you supposed to remain stoic if you become overly sympathetic? I'm immensely sadistic, and sometimes even I have to take deliberate measures to disregard a boy's plight if I'm taking him especially far. The easiest way for me to process it is to treat it like having a POW - except this person is actually electing to be dehumanized in some manner. If you have a good enough relationship with a boy to push him to that degree, it really can mean actively ignoring sympathy in order to further the dynamic. It takes some work on both parts, but once the boy's headspace is there it can be both incredibly hot and fulfilling.