There's just something about whump scenarios where the whumpee who ends up in the whumpee's crosshairs isn't even their main focus. Hero taking on the nemesis of a team member, friend in wrong place at the wrong time, whumper specifically targeting their former whumpee's current circle. Because there's an intriguing asymmetry to it, and there are so many layers. The one being whumped isn't even important—and what's worse, being the object of obsession for a sadist or being incidental and disposable?
But then there's the past trauma reveal potential. Because maybe they know what the former whumpee went through at the hands of this person—and so are boiling with a potent fury at what they've done threaded with fear for the knowledge of what they're capable of—but maybe this is how they find out. Maybe they realize on their own what must have happened and the horror rises like bile. Maybe whumper tells them, bragging, taunting, gleeful, possessive, matter of fact, solemn but unremorseful. And then if the former whumpee gets involved (rescue, negotiation, sent a message), maybe they're forced to watch, or they can see that current whumpee knows now by how they look at (or can't look at) them, or whumper tells them.
And now, whatever happens, they have this terrible shared understanding, but they experienced it so differently. Maybe whumpee says why didn't you tell me. Maybe they say I thought I understood but I had no idea. It changes things between them.
And in the end, whumper is still only truly interested in the former whumpee, who knows that whumpee would never have crossed paths with this person if not for their connection.