Prince Humperdink, in William Goldman’s classic fantasy romance novel The Princess Bride, is certainly a sadist. The definition of a sadist is a person who derives pleasure from the pain and humiliation of others. At a passing glance, Prince Humperdink might pass of as pragmatic: he has a specific goal (marrying Buttercup) and will stop at nothing to achieve his goal, in a calculated, almost utilitarian fashion. What puts him over the edge, however, is the pleasure he derives from controlling the lives of the other characters and the extra measures he enjoys going to past the point of pragmatism.
More specifically, Prince Humperdink is a Tyrannical Sadist. Tyrannical Sadists are often people in positions of power, and rather than being openly violent or motivated by mistreatment, they relish in the total subjugation of the spirits of their victims. Basically, they are more concerned with breaking the will of a person than breaking their bones.
Prince Humperdink is a Tyrannical Sadist because he is after something he cannot attain through brute, physical force: namely, the love of Princess Buttercup. He crosses the line into sadism when he orders Westley to be executed after swearing on his father’s life and his mother’s spirit that he would go unharmed if Buttercup agreed to marry him. He wants to completely subjugate Buttercup as a wife and Westley as an adversary. Thankfully for the story, he fails.