Whumpees who feel rage inside, rage about what's done to them, about who did this to them, how they had no right, how this shouldn't have happened, how something has been taken and they can't get it back. They feel rage about their rage, about how this... anger, this monster, inside them has been created, and how they can't get it out.
They find Whumper. They fight them, and beat them. And as they're beating Whumper who has gone down (maybe they're already dead, maybe not) they break down. They started out angry and snarling, but now they're just crying and sobbing, until they scream, letting out all their rage.
It's Whumper's fault, it's all Whumper's fault, and now here they are, bloody, violent, and broken.
So they stop, and just sob. They're not angry anymore. Just empty.
So, so empty.