But even in universe tieflings are not evil by nature... And it's supposed to be common knowledge isn't it? How is it any different from killing everyone in a village of humans that some of them are evil?
Well I disagree that a race descended from a race of evildoers is not evil by nature. That makes perfectly zero sense. That's like saying there is nothing elfy about half-elves. I mean maybe there really is no difference between humans and halfelves and tieflings other then appearances, but that's actually bad design in my eyes. Tieflings shouldn't just be "humans with horns and tails"