There is a surprising amount is philosophy on bdsm and the consensus is that the sub, even though they take the role of submissive is actually dominant, as a real sadist would not enjoy the other person enjoying themselves from the suffering they cause, and thus the dom isn't a sadist, they're a regular person being dominated by a masochistic person or a normal person who gets off to the adrenaline and isn't really a sadist. A regular sadist would want 0 limitations.