Below in all of it's bad grammar glory is something I posted on Facebook. I'm not typically one to cross post stuff from Facebook on my blogs because I'm not comfortable translating things from different platforms. This, however, is one of my exceptions. Before I share it, though, please allow me to provide some context.
In a writer's group I'm apart of on Facebook, a discussion arose as to how to create a truly scary antagonist. Several people bemoaned the difficulty of providing a good scare when so much seems to be based in viewing fear as directly proportional to the amount of gory violence detailed. (Which is a reasonable complaint that is especially valid with respect to the visual medium such as television or movies.) Somewhere in the course of the discussion, someone tossed out how sociopathy is a 'scary' concept.
Sociopathy as a 'scary' trait is a heavily over used trope, in my opinion. It has become short hand for saying that a character is a villain, along with things such as bipolar, mania, and schizophrenia. This trope is so prevalent that it is accepted as common knowledge and bandied about in the media as an explanation for things such as mass murder. I have a justifiable hatred for this trope. It comes from multiple things. But, I think that this bit that I am cross posting from Facebook makes my position clear, so let me present it now.
i prefer it when the sociopath trope is turned on its head. when the hero is one, things get much more interesting in my opinion.
also, the argument that they are inherently amoral is not really a good one to go with. there is a significant number of people in the general population who have a sociopathy diagnosis that are not dangers to society. they do tend to be very stoic and their reasoning as to why something is appropriate or not tends to be based on logic or a code of honor they choose to adhere to rather than emotion.
honestly, the idea of using mental illness as a stock trait for your antagonist can be slipshod and poorly done if you're just going to plug in what you find on the internet describing symptoms instead of building the character so that their symptoms are not their only 'scary' quality. stereotyping mental illness as the prime motivator of a villain's behavior is piss poor writing, in my opinion, and only furthers the negative stigma that people in the real world face as suffering from mental illness. it is just as bad as making your villains of an ethnic minority simply because you need to shoehorn in a minority character and some how provide an additional dimension to your character.
now, mental illness CAN be a very powerful factor in an antagonist's motivations and scare factor but it requires a degree of sensitivity to things such as how mental illness effects all parties in a situation. exquisite horror can be written showing how your sociopath character is suffering due to their illness with out removing the horror of their deeds. i can tell you that the fight with your own mind is just as terrifying as having someone directly threaten your life, especially when it is a 24-7 experience with no relief.
also, another reason to consider showing your antagonist's suffering from their illness is because it humanizes them and effectively forces your reader to view them as a person rather than a straight up monster. this plants that seed of identification with the antagonist which makes the horror of what they do that much greater because a part of the reader is forced to envision themselves enacting it.
tl:dr - mental illness does not equal good villain. please tread carefully, do your research, and make it one of many identifying traits rather than the primary trait that marks them as an antagonist. failing to do so makes it harder for people with mental illness IRL.