A recurring and abstract theme in fiction is that homogeneity (consisting all of one thing or quality) is good, while heterogeneity (consisting of multiple things or qualities) is bad. This manifests in many forms, both In-Universe and out, and while it doesn't always feed into a coherent Aesop, it often carries the underlying implication that, well, heterogeneity is evil. Of course, it may also be used to carry the opposite aesop—for instance, if the half-elf Big Bad is a Woobie, Destroyer of Worlds because of Half-Breed Discrimination, the work may convey an anti-racism message. When played straight, this generally falls under the "Homogenous Heroes vs. Heterogenous Villains" variant of Alike and Antithetical Adversaries. This concept often shows up in the following specific forms:
- Half-Breed Discrimination: An In-Universe variant, when the mixture of two races (one of which is usually human) is seen as an inherently bad thing (which in itself is another example of this concept), and the offspring of such relations is therefore anathema. Half-elves are probably the most commonly shunned half-breeds in fiction, but there are also the half-vampire Dhampir, the half-angel Nephilim, and more. Needless to say, this line of thinking has its roots in Real Life proscriptions against mixed-race relationships, which means it may (accidentally or deliberately) deliver a pro-racism Aesop, but it's predictably common to see this variant used to deliver an anti-racism Aesop instead.
- Magitek is Evil: Pretty much says it all; the act of combining science with magic, especially technology with magic, is considered evil. Why? Pick your favorite explanation. Maybe God said not to do it. Maybe the combination is just too powerful for any human to be trusted with. Or maybe it's just inherently evil and no reason is needed or wanted.
- Bio-Augmentation is Evil: This one is essentially Technology+Biology. Using technology to enhance, alter, or fix the human body (or any sentient species, depending on the setting) is evil or at least dangerous. This can be as reasonable as "Don't put your Brain in a Jar," or it could be as restrictive as "Don't use prosthetics," and in some cases it can even go as far as "Don't use modern medicine," in which case it's less about cybernetics and shades into the more general Science Is Bad. Many of these works, though, won't actually condemn science as a whole, but will instead focus on the evils of using science specifically when meddling with the natural state of the human (or other sentient species) body, which is what makes it an example of this concept. A key trope of this variety is Cybernetics Eat Your Soul, in which a person who has been augmented will turn out to be evil. When an Evilutionary Biologist is making Mix-and-Match Critters, it often provokes cries of Playing God; likewise, Hybrid Monsters are generally evil or at least antagonistic, and Bio-Augmentation is usually portrayed as ethically questionable at best.
- Split Personality: In itself this isn't necessarily an Evil Trope, but whenever a character has a Split Personality, Two Beings in One Body, or (most obviously) a Superpowered Evil Side, one or more of their "personalities" is almost invariably evil or at least highly unstable/insane/dangerous.
Hello, Unknown Troper. You'll need to get known to lend a hand here.