>Razing Hell: When Genocide is the Answer
Sometimes in history there would come a great villain who just didn’t get with the program. The Classical example is the Assyrians. Those bastards went around from city to city stacking heads in piles and levying 100% taxation and such to conquered foes. They became. . . unpopular, and eventually were destroyed as a people. That’s the law of the jungle as far back as there are any records: if a group pushes things too far the rules of mercy and raiding simply stop applying. Goblins, orcs, sahuagin. . . these guys generally aren’t going to cross that line. But if they do, it’s OK for the gloves to come off. In fact, if some group of orcs decides to kill everyone in your village while you’re out hunting so that you come home to find that you are the last survivor, other humanoids (even other Evil humanoids like gnolls) will sign up to exterminate the tribe that has crossed the line.

Cultural relativism goes pretty far in D&D. Acceptable cultural practices include some pretty over-the-top practices such as slavery, cannibalism, and human sacrifice. But genocide is still right out. That being said, some creatures simply haven’t gotten with the program, and they are kill-on-sight anywhere in the civilized world or in the tribes of savage humanoids. Mindflayers, Kuo-Toans, and [Monster] simply do not play the same game that everyone else is playing, mostly because their culture simply does not understand other races as having value. And that means that even other Evil races want to exterminate those peoples as a public service. Like the Assyrians, they’ve simply pushed their luck too far, and the local hobgoblin king will let you marry his daughter if you help wipe them out of an area.