Misunderstandings: Big Top. Torturing animals (both wild and sentient) in his sideshow for fun and profit is a dead give away. He also extremely manipulative, blackmailing a griffin into entering his exhibit, and torturing her into acting vicious. And he lied to her about her father being dead for several months and threaten to break her completely, like he had done to a minotaur, all with a huge grin on his face.
In NightbladeNick claims to be one. Whether or not he is, he has many traits of being a sociopath, including the lack of moral compass and empathy.
Lampshaded in No Hoper when one of the vampyre students dies in front of Light and he shows no emotion over her death. The other students whine and scream at him and call him a sociopath. Light only agrees and tells them that they're probably right.
In Perfection is Overrated, Hitomi Kirihara, a Deconstruction of a Jerk Sue, has this as her personality. She sees nothing wrong with using her Mind Control powers to rob and later murder people, and cares for no one other than herself to the point at which when her Child is destroyed, which would normally result in the death of the person she valued most, Hitomi herself dies.
Uxie from Poké Wars shows all the signs. Utterly incapable of feeling remorse. Treats everyone around him as little more than disposable lab equipment. He casually orders genocide with the ease that someone would order a pizza.
In Prison Island Break, the writer stated determination that Big Bad Mephiles be a pure sociopath - while the criminals should be seen to have sympathetic qualities as they developed or their pasts were revealed to have twisted them to such a broken point, there must be nothing redeemable about Mephiles whatsoever. He has no distinct hardships in his past, but revels in his power to cause suffering whether in criminals or innocent staff.
For example, Sonic cares for his whole gang and is always deeply troubled when they get hurt, even if non-members don't concern him much, while Shadow develops a degree of protectiveness towards the weaker Silver and comes to respect Sonic. Even Scourge suffers from a drug addiction that was ultimately implemented by Mephiles himself.
The original Falla Cii has shades of this. Intelligent and manipulative? Check; she managed to easily manipulate Kyouko and ultimately trick Tsukune's group, who know from what Falla's good sister Luna told them that Falla is a heartless bitch, into thinking she wants to be come a good person? Complete Lack of Empathy and moral conscience? Check; Falla went so far as to send her own little sister Complica to her death For the Evulz, and later outright admitted to Luna and their mother that she viewed Complica as a disgrace to their species and she's happy that her younger sister is dead. On top of it all, she comes right out and says that nothing matters to her more than herself and power, not even her own family, and goes so far as to mock Tsukune and co. for ever believing that she would "choose family over power."
Tsali the Ultimate Weapon from Sonic X: Dark Chaos is a Deconstruction of this. Several systems in his artificial brain were secretly modified to completely suppress his empathy and sense of mercy so he would never feel remorse for his crimes. However, when those systems are accidentallydamaged...
It's debatable if Theodore Nott is an example or not in To the Continuation, though, one character does explicitly label him as such, and he seems to agree. However, aside from one murder, which he had a somewhat sympathetic motive for, he doesn't really do anything concretely bad during the story.
Cedric from Voodoo's Disciple is considered to be one in the the story. Despite this he thinks that psychopathy fits him better.
Harry Potter ends up a sociopath in For Love of Magic and openly admits that it's only the women in his life arguing against it that keeps him from kidnapping and murdering the children of Death Eaters before they can become a problem. He uses them as a Morality Chain as he long since lost any morals he had and while most of his actions are part of an end goal, sometimes it's just for the hell of it. More than once he refers to or thinks of people in general as being useless bags of meat. His reasoning for not executing Voldemort's captured followers immediately? It's less wasteful to use them for human experimentation first.
Later chapters show that his lovers have started showing shades of this. When Harry discusses the concept of "The call of the void"note Stray thoughts, often focused around committing suicide or murder, usually associated with the idea of jumping off a ledge, Tonks admits to herself that these days, she mostly advises moral choices out of a combination of habit and the belief that someone should.
Melanie Sampson in the Twilight fanfic For You I Will is this. Unfortunately, Embry imprinted on her, meaning she now has a werewolf who is incapable of disobeying her as her accomplice.
Benjamin Hares, the estranged husband of Grace Glossy from Old West. When he and Grace were young, he charmed Grace to marry him against her father's warnings. Though Grace was devoted to him, he tended to guilt-trip her whenever they argued. One year following the marriage, Benjamin was caught up by the debts he had hoarded as a conman, so he sold everything he owned with Grace and skipped down, leaving the pregnant Grace no choice but to return to her father. Ten years of absence later, Benjamin's in debts again, and Dufayel offers to cover him up in exchange for selling Grace's property to him as her husband. He attempts to use his old tactics of flattery, guilt-tripping and feigned concern, but the resistance he gets from Grace and their son Teddy chafes Benjamin who dares to be angry at Grace when she refuses to allow him to sell her out again. He asks her to consider his situation, but he doesn't do likewise with her, even daring to claim that he left because she never loved him enough. When she decides they're done for and tries to throw him out, he beats her up without any sign of shame, leaving her traumatized by the violent side he has never shown her before.
The RWBY fanfiction series The Makings of Team CRME has a lot of them. Aside from the canon characters (Cinder, Mercury, Roman, and Adam), each origin story has at least one.
Ruby Rose is a serial killer with an ego only overshadowed by her bloodlust. Desiring to surpass gods by becoming the most evil being in existence, she violently slaughters her own parents, mutilates her older half-sister Yang Xiao Long by cutting off her arms and gouging out her eyes, then travels around Remnant as a Serial Killer. When Vale is bombed by Lord Ozpin, Ruby waits by escape routes to kill fleeing survivors. Even after the carnage, she sticks around the ruins just to terrorize the survivors.
The aforementioned Lord Ozpin is a ruthless tyrant who demands respect and seeks power at any cost, ruling a dictatorship over Remnant and forming elite armies of warriors to snuff out any resistance and ensure his place of power. He bombs Vale as a demonstration of his power and authority, and otlaws anything that displeases him, including romance in the ranks of his enforcers.
Takeshi Yamamoto from Lightning Never Strikes Twice is acutely aware of the fact he's a disturbed kid. Since he was five years old, he found other people boring and dull, so he basically forces himself to be charming and amiable, which he pulls off flawlessly, but pretending to be the perfect Nice Guy doesn't stop him to do rash thing - such as trying to jump off the school roof - or emotionally manipulating his father whom he thinks is unbearably sentimental. He's quite unusual for the mould since he genuinely wants to feel something, but it's ambiguous if it's genuine longing for normality or merely boredom driving him crazy.
Like many mental illnesses, Takeshi's problem is In the Blood, with his maternal uncle Eishun Kobayashi as a lower-functioning sufferer. Man actually killed his big sister's pet cat at six years old to feel something - he didn't - is unable to emote without giving a "plastic" impression and works as a hitman because that's the only thing he's good at doing. Needless to say, Takeshi and his late mother are quite disturbed by the possibility of the youth turning out like Eishun.
Agent Florida also shows signs of this. He often acts like aNice Guy and can almost be genuine about it, but he's perfectly willing to kill people whenever the situation calls for it. He has a genuinely friendly conversation with a guard that he was assigned to take out and when the guard figures out that he doesn't work there, Florida kills him without a second thought and never loses his cheerful attitude. He even mentions the fact that the guard's daughter will never see her father again like it's a mild inconvenience.
Nabiki Tendo in A DIC-less SI only cares about making things benefit her. Not only does she trick her sister into attacking an untrained civilian twice to steal a magic ring off him, but when Akane seemingly kills said civilian, Nabiki's only concern is how she can benefit from it or at least make sure having a murderer as a sister won't harm her finances.
Prince Jewelius from the My Little Pony: Friendship Is Magic fanfic Loved And Lost. Initially he's presented as a nice and humble stallion, but after he steals Equestria's throne from the alicorns by using the events of "A Canterlot Wedding" to his advantage, he shows himself to be a totally selfish and conceited tyrant who's willing to cause death and ruination out of spite or simply to get rid of few ponies. A skilled manipulator, he tricks Twilight and the entire city of Canterlot to turn against the alicorns as well as Twilight's brother and friends, and he previously tricked Queen Chrysalis, another skilled manipulator, into trusting him so much that she didn't expect him to double-cross her. He has for years secretly hated his aunt Celestia and cousin Cadance (who regards him as her younger brother) simply for being more popular than him. He takes delight from the pain of his enemies, both physical and psychological, and he stubbornly regards his tyrannical rule to be glorious and superior to that of Celestia. His heart is devoid of love so much that Cadance's love spell has no effect on him, and the Changelings aren't able to drain love from him.
The Bloods of Bolton is basically a study on anti-social personality disorder in a specific bloodline. Frankly, it's a miracle that Domeric grew up with morals.
Roose is extremely cruel towards his servants when they screw up, refuses to emote no matter his actions and is unperturbed by his daughter's Troubling Unchildlike Behaviour.
Drucilla doesn't see people outside her family as worth caring for and has a nasty tendence towards Disproportionate Retribution — sewing the kennelmaster's daughter's mouth shut for badmouthing Roose, something she practiced with her dolls. She started when she was barely eight-year-old.
Ramsay is the worse of the lot: he already was a Serial Killer at twelve-year-old and feels no remorse in terrifying and potentially murdering his siblings.
Vale's Underground has many sociopaths, but the apex example is the mob boss only known to most as "Cinder." She is an almost textbook example of the trope. She has extreme callousness towards others and is willing to use violence whenever she can get away with it. She admits that she likes Mercury (another sociopath) because he doesn't have any moral hangups, thus proving how true this is for her. She can be superficially charming either when necessary or to mock someone when she's torturing them. She also seems to have a need for stimulation that is expressed through her sadistic urges and occasional sexual relationship with Mercury. Cinder does have some anger issues and a need to be violent with others along with some arrogance about being the most prevalent mob boss in Vale. She also gets manipulative by seducing Emerald and blackmailing Taurus. Her mother, Salem, notes that she always had signs of this even in her youth. Even acknowledging that Cinder doesn't even love her.