Cutting down all tieflings on sight sounds very much like the kind of things a racist, narrow-minded, righteous paladin or other noble knight could very much do (though that's certainly not the kind of paladin I intend to play). Of course, it is evil, from our point of view as players. But for a certain type of character, that would be thought of as a very good deed. In fact, I would see this type of slaughter coming from a misguided good character a lot more easily than a murder-loving-muhaha-so-evil character who thinks they can get away with that since nobody will stand for tieflings anyway.
