Atreus might win the award for most messed-up revenge ever when he tricks Thyestes into eating his own children, but it's not like Thyestes is a particularly mellow dude either. Along with Atreus, he murders his half-brother, Chrysippus, at a young age, and then proceeds to do all kinds of other jaw-droppingly awful things. Thyestes starts the feud with his brother by sleeping with Atreus's wife, stealing his golden fleece, and trying to take the throne of Mycenae. It's pretty hard to justify Atreus's particular brand of bloody justice, but there's no getting around the fact that Thyestes brings his misery on himself.
Later in the story, Thyestes manages to find even more awful things to do. An oracle tells him that if he has a son by his own daughter, Pelopia, that son will kill Atreus. So, Thyestes then proceeds to force himself on his daughter and impregnate her. In some different versions of the story, Thyestes doesn't receive a prophecy and rapes Pelopia not knowing that she's his daughter at all. Though this scenario does make Thyestes an unintentional incestuous rapist, he's still a rapist. No excuse for that. In the end, both Thyestes and Atreus show themselves to be ultra-brutal human beings, and one can't help but think that the myth-makers who came up with this legend are holding both brothers up as examples of what not to do. (Or at least, we hope so.)