Mandy and Cameron Klinefeld from season 4's "Assume Nothing" put the "unholy" back in Unholy Matrimony. A married couple united by a shared sexual fetish for rape and murder, the pair operate by abducting other husbands and wives, sexually torturing the women while making the men watch, with the ultimate goal of forcing them to Mercy Kill the women they love. They promise the husbands they can go free if they do this, but they're lying.
Season 12 Arc Villain Laura Gabriel is a ruthless arms smuggler and Big Bad Friend to Catherine Willows. Playing a meek and submissive wife to her sleazy private military firm head husband Mark, she uses this false personality to deflect blame off her and onto him as she funnels his company's weapons to The Mafia and The Cartels. When the investigation starts getting too close to the truth, she hires employees of the company to murder her own lawyer and all of his staff; following this, she kills the assassins and burns their bodies beyond recognition in order to fake her own death and that of her right-hand man. She intended for an enraged Catherine to summarily execute her husband, and placed a contract on Catherine herself to clean up the last remaining loose end, not caring for her old friend in the slightest.
Josh Freely from season 1's "Fly Away" is a brutal pedophile that uses his position, as a social worker working with emotionally unstable parents, to find new victims. Freely piles emotional abuse onto the parents, making them feel as though they're worthless, so they surrender their kids to him, and if that doesn't work, he's more than happy to fudge his own records so his superiors order the children removed. His abuse eventually reaches the point that one such mother attempts to kill herself and her daughter rather than let Freely take her.
The season 1 finale, "Lovers' Lane, " brings us Jim Larkin, a slovenly glutton and serial rapist. Too lazy to even abduct victims himself, he instead badgers his abused and weak-willed son to do so for him, usually unattractive teenage girls he pretends to befriend. His assaults are brutal, reaching the point at which he murders one of his victims simply for calling him "disgusting."
The unnamed killer going by the alias "John Smith" from season 5's "The Road" is the most horrific Serial Killer seen on the show. Disturbed since childhood, he described the sight of watching a woman drown while doing nothing to save her as the most beautiful thing he'd ever seen, and dedicated his adult life to replicating that "beauty." Abducting women who were perfectly happy with their lives, Smith brought them to special cellars where he sealed them off, watching as they went insane from isolation before finally leaving them there to starve.
Rayanne Leland from season 5's "Spiders" is a sugary-sweet stay-at-home mom who also happens to run a Neo-Nazi coven out of her basement. When her son Truitt murders a Hispanic woman, Truitt's girlfriend, Tamyra, turns to Rayanne for help, only to find to her horror that Rayanne wholeheartedly supports her son's actions, and calmly tells Tamyra that all Hispanics should be exterminated, her warm, loving smile never leaving her face. When Tamyra threatens to go to the police, Rayanne browbeats the most insecure and sympathetic member of the coven, Elliot, into murdering her.
Daniel Patterson from season 5's "Slipping" is the ultimate gaslighter. Insanely jealous of his wife's skill at poetry, he hatched a plan to both steal her work for his own and get rid of her. With the aid of his slavishly devoted housekeeper, he did everything in his power to drive his wife insane and ultimately to suicide, maintaining a fašade of a caring husband all the while. When he failed to break her, he murdered her himself. To prevent his stepdaughter from knowing the truth he sent her to a notoriously harsh boarding school, believing his story that her mother committed suicide for 45 years.
Season 3's finale and season 4's premiere had Emil Dornvald, a mercenary working for an African dictator. The missing person of the week is in love with a rebel opposing said dictator and knew where he and his friends were holed up. Dornvald cold-bloodedly murders the pregnant teenager she's been mentoring and is very fond of, sends her a picture on the girl's phone, and offers to stop killing her loved ones if she just tells him where the men are.
Season 6's premiere had a conman preying on families who had adopted children from Africa; he'd claim to be the kid's birth father and guilt the parents into surrendering the kids to him, whereupon he'd take them and sell them on the black market. He also manipulated a Sudanese war orphan into helping him by claiming that he was only stealing the children to get them back to their birth parents, the young man having lost his own mother to militants. When the orphan finds out he's been had, he brutally stabs the guy to death and the FBI quite understandably decide to look the other way.