I've been involved with several SOs that had sadistic tendencies, and really seemed to enjoy hurting people. It always seemed to me that they were reversing their role in prior relationships, to take back power and feel in control, doing to me what their ex had done to them.
This really enrages me, because it's hypocritical, makes no rational sense. How can you complain about how you've been treated and then turn around and do it to somebody else?
And while being mistreated or abused makes me want to see them suffer, I don't want to be the source; I want karma to kick their ass. Probably because I think what most of us want is for our abuser to understand they've done wrong and be genuinely sorry.
Inflicting revenge negates that possibility, because the abuser feels retroactively justified in whatever they've done: "You retaliated against me for hurting you, which means you deserved it in the first place," never, "Oh, I guess if I hadn't done those bad things to you, you wouldn't be responding in kind."
Also, I don't want to be anything like those people.
It's hard, though, not to feel some satisfaction if, say, your ex screwed you over and you find out their new partner is screwing THEM over.