I’ve noticed a tendency for sadists to respond poorly to weakness in others (to put it mildly). They appear to be unable to experience anger in a non-sexual way, which comes out as disgust (the sense of being vertically “above” the disgusting object). The mix of anger and disgust is contempt, so the result is for the sadist to bully the fetishized object until it no longer exists to threaten the in-group (that is, it removes itself from the in-group or dies). This would explain the predictable behavior of labeling the bully’s victim as something contemptible (nowadays “Gamma” if male, “slut” if female), and the existence of snuff pornos. The masochistic analogue would be anger*worship = hatred.
I noticed that sadomasochism (like all fetishes) is a form of masturbation where another person (or thing) acts as a dramatic prop for the misdirected behavior. E.g. The woman reading 50 Shades of Gray anthropomorphizes the desire to “beat herself up” as a man who wants to beat her up.
So where masochism anthropomorphizes self-hatred, sadism anthropomorphizes self-contempt. That is, at a deeper level the sadist wishes to be set free from his painful weakness, just as a masochist wants to be set free from the responsibility to overcome, defeat the Other, and win. This self-contempt is projected onto the fetish: if weakness deserves to be punished, then the victim ought to thank the bully for purifying their disgusting sins by inflicting struggles on them, which ought to train up the sadist’s “will-to-power” ethos within them. This unconscious purpose is never realized, but this only strengthens the perversion because the unfulfilled desire remains and the original channel is broken.
In any case, the act itself is considered morally self-justifying because if weakness and sin are the same, then anyone who is a victim deserves it by definition (the opposite of victim morality).