This thread reminds me of the Ilwrath from Star Control 2. They are a truly evil society, torturing and eating all sentient beings as sacrifices to their dark gods, burning any world they can get their spidery appendages on, causing fear for its own sake, and so on. The player can go to their homeworld and engage them in a fun ethical debate, arguing that because their society takes evil as the cultural ideal, and because evil is a concept that is subjective to the values of the society in question, are they not in fact being good by doing evil as they ought to do?
At that point they get really confused and angry, then attack you.