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Tropes are storytelling devices. This series of articles on how to use or not use them in comics was first published in my Comics & Cartoons Weekly on deviantART.
Always Chaotic Evil
This is when a story contains an entire race of bad guys, a common case of bad writing. Not a case of relativity where each side thinks they are the good guys and the others are bad – no, these people know they are evil and may even brag about it. The whole culture somehow agreed to all serve the same evil purpose (because it's easy in real life to make a mass of people agree on one thing, right?)
This was seen so much in the past, in all media, and the trend has not quite disappeared, though nowadays [open] racism is decidedly out of favor, so the evil races tend to be alien and no longer human*. One thing worse than having an entire evil race, is to have an entire evil race with one or two "good" exceptions. This is patronizing at best, and no way to create three-dimensional characters.
[* Exception made for Nazi Germany,which seems to still be a convenient nest of evil to use, as there is a not-so-unspoken consensus that this is noncontroversial to the vast majority (not that I agree or disagree with this, but from a storytelling perspective it's getting old).]
In this day and age, with the world at our fingertips making us all much more aware of shades of grey, it is nearly impossible for any writer to get away with such a characterization unless it's framed in a convincing way. Without drifting into politico-social issues, the question that interests us is: How do we avoid Always Chaotic Evil when our story tends to head that way? The simple answer is also the answer, I think, to most writing-related questions: look at real life and historical examples. There are plenty of reasons why a human group can be perceived as, or seem to act as, a single-minded entity, without actually being one. These real-life situations may inspire you solutions to fit your story.
• An example that deals with perception is that of the biased narrator: Any chronicler who ever wrote about his nation's enemies, from Julius Caesar to WW2 Russian columnists, has demonized them and so that it becomes impossible to think of them as fellow human beings. A story explicitly told from such a subjective point of view can get away with any amount of this, provided you make it very clear that this is a case of biased narrator. 300 makes use of this approach in its portrayal of the Persians.
• Another example where a population can, as a whole, be unsafe to outsiders is that of a population under dictatorship. Such regimes imply secret services so that it is highly dangerous to speak against the regime or have any contact with its enemies (cf. Myanmar, Stalinian Russia). An outsider has to take care not to speak against the regime, and an "enemy", if found out, would be promptly "disappeared" – as would anyone they dealt with. This is a real-life situation where the danger has nothing to do with personal evil. Plenty of plot-driving tension can be derived from such a reference. Looking into religious taboos, caste systems, propaganda, tribal warfare etc can also yield different approaches.
These were just suggestions off the top of my head. The most important thing is to take a careful look at your story if you think you may have erred in this direction, and do what's appropriate for your plot.