Complete Monster/Sword Art Online

"Johnny Black: Let's play the game where they have to kill each other! Last one standing wins! PoH (no angel himself): I know it's your favorite but last time we did that you killed the winner anyway. Johnny Black: Oh come on! You spoiled it! It's no fun if they know they're gonna die!" Sword Art Online features some rather irredeemable villains.

Fairy Dance Arc

 * Suguo Nobuyuki (Oberon the Fairy King) is the director of research and development at RECTO Progress Incorporated, and the one-time fiance of female lead, Asuna Yuuki. He was also completely obsessed with research into the manipulation of human emotion. Following the clearing of Sword Art Online, Suguo trapped three hundred of the surviving players in his own game, Alfheim Online, where he ran experiments on their minds, testing their fear and pain responses. Kidnapping Asuna in the game, he keeps her trapped in a gilded cage at the top of Alfheim's World Tree, while plotting to wed the comatose girl in the real world. When Kirito reaches the top of the World Tree, Suguo uses his administrator abilities to paralyze him, tortures him, and then sexually assaults Asuna in front of him, gloating the entire time about how, once he's freed, he will rape her again in real life. When the plan fails, he attempts to kill Kirito in the real world, attacking him with a knife. In the context of the story, Suguo is as bad as it gets.

Phantom Bullet Arc

 * Red Eyed XaXa (real name: Shoichi Shinkawa) is one of the top members of the Laughing Coffin Guild in SAO, and the Guild's Master Swordsman. During the Murder Investigation Arc in SAO, XaXa, alongside PoH and Johnny Black, ambushed and attempted to murder players Caynz, Yolko, and Schmitt; when Kirito intervened, the trio also tried to kill him. After making his way back to the real world, XaXa, bored, and seeking a way to kill again, took advantage of his younger brother Kyoji's fragile mental state, to convince Kyoji to create the online persona of Sterben/Death Gun in Gun Gale Online, ultimately hijacking Kyoji's desire to be "the strongest" for his own ends. Using Kyoji, and his old partner Johnny Black as aides, the psychotic XaXa was able to arrange the murders of players Zekushiido, Usujio Tarako, Pale Rider, and Garret, before sending Kyoji to murder his own crush, Shino Asada. A sadist at heart, XaXa makes a point of crippling his opponents' avatars with a paralyzing toxin before delivering the coup de grace, and during his Sword Fight with Kirito in GGO does his best to cut him to ribbons, cackling madly and mocking him the entire time.
 * Atsushi Kanemoto, aka Johnny Black, is another top member of the Laughing Coffin Guild. As the Guild's Master Poisoner, Johnny came up with a way to paralyze opponents so that they could then be tortured to death, and puts this poison into effect during his, XaXa, and PoH's attack on the members of the Golden Apple Guild. When discussing what they will do with the captured trio, Johnny expresses a desire to make them fight each other to the death, promising to release the survivor; when PoH notes that Johnny will just kill the survivor, Johnny complains about how the Guild Leader has ruined the surprise. He subsequently tries to poison Kirito when he comes to the Golden Apple trio's rescue. Escaping into the real world, Johnny was recruited by XaXa during the Death Gun murders, poisoning Pale Rider and Garret in their homes, before again making a getaway. He was finally arrested when, after having lain in wait for six months, he attacked Kirito and Asuna on the street, intending to inject them with poison from a syringe, and hospitalizing Kirito. Unable to quit killing in the virtual world or the real world, Johnny is the work's definitive Psycho for Hire.

Alicization Arc

 * Vassago Cassals, aka PoH (short for Prince of Hell), was the psychopathic founder of the aforementioned Laughing Coffin Guild, where he rallied together the worst of those trapped in the game—including the aforementioned XaXa and Johnny Black—and had them slaughter their fellow players, all for the joy of seeing East Asian players kill each other. Later betraying and abandoning his own Guild to die, PoH corrupted several other players and sent them to kill those who had participated in the Laughing Coffin's downfall. Biding his time after being released from the game, PoH returned to torment Kirito again, brutally beating him and promising to torture his lover Asuna to death. After being defeated, POH asks for a "warrior's death," while planning to use being ejected from the game as a chance to kill the helpless Kirito and Asuna.
 * Quinella is the ruler of the Underworld server and High Minister of the Axiom Church. In an effort to obtain immortality, she fused herself with the Cardinal System of Underworld. After her success, she began corrupting the souls of the Underworld nobility by separating the nations and spreading ignorance and fear among the populace. After making the populace believe that she was a goddess, she created the Axiom Church and named herself the Highest Minister. Quinella brainwashed every rebel fighting against her government and the Champions of the Four Empires Unity Tournament. She also performs fatal experiments on innocent people in order to learn the secrets of the human units. After realizing the invasion of the Dark Territory was inevitable, Quinella confiscated every line of defenses from Human Empire in order to make the people more dependent on the Axiom Church. When faced with possible defeat by the Dark Territory, Quinella transforms thousands of people (including familiars of her own Integrity Knights) into swords and forms a Golem out of them, her victims conscious in their new form. She claims this action was for the safety of these people, but it's clear she shows no remorse for trapping these people in such a state. Quinella is ultimately a narcissistic sociopath who only wants to control everything in her reach, killing and torturing all who dare challenge her.
 * Gabriel Miller, aka Subtilizer in GGO and Vector in Alicization, is the Big Bad of the second half of the Alicization Arc. A cold sociopath even as a child, Gabriel manages to assault the Ocean Turtle and enter into the Underworld in the super-account of Vector, god of darkness, intending to capture the only fully-developed Fluctlight and brainwash her into be his submissive wife and "start a new life in a new world". With his new godlike powers, Gabriel turns the turbulent Seven Lords Assembly of the Dark Territory into his personal dictatorship, taking advantage of the flaws in the data of the habitats, who are forced to follow the "rule of the strongest". When the wall of the Human Empire colapses, Gabriel leads the invasion, using PoH and DIL as his Co-Dragons. When the invasion doesn't go as he expected, Gabriel decides to use more risky strategies, causing unnecessary deaths on his own side, as he believes that the sheer numbers and experience of his side are the only things he needs to win, going so far to use members of his own army as Demihuman sacrifices to empower DIL and sending his melee fighter to cross an abyss on a rope, all of this without care for the lives of "the uncomplete Fluctlights", or souls. On top of that, Gabriel goes so far as to think of the possibilty of destroying the Underworld and killing every IA in it after his victory. Gabriel Miller is the epitome of sociopathy and selfishness in the entire SAO series, becoming the series' biggest Knight of Cerebus along with Suguo. Quinella might be caused the biggest damage to the society of the Human Empire, but no one had caused as much death, misery and destruction in the Underworld than Gabriel.