Constantin Volsky, the Russian mob boss in the two-parter "Refuge". He ordered an innocent man murdered and dismembered because he had a relative who wouldn't go along with a money laundering scheme. When he finds out that the key eyewitness for the prosecution is a six-year-old boy, he sends assassins to kill him. They murder his mother and an A.D.A. The boy lives, but only because they didn't cut deep enough into his throat. In the second part, he orders the bombing of the precinct where the police protagonists work, just for giggles.
Leon Vorgitch from "Deadlock" was in prison for killing five people. He then kills two guards and escape from prison. When the detectives come for him it results in even more deaths, including a classroom of children. When Detective Green asks Vorgitch why he killed the kids, he laughs and says "Why not?" In the end he brags to Jack and Connie that he will escape again and kill both of them.
Mark Bruner from the season fourteen episode "Bodies". Raped and murdered no less than 17 teenage girls and kept most of their bodies rotting in an undisclosed location so he could go and admire them as they decayed. He terrified his original defence attorney so much that McCoy helped to get her reassigned, terrified Serena, visibly unnerved Briscoe, Green, and McCoy. What made him truly monstrous was his refusal to reveal the location of his dumping ground so that his victims might be identified, specifically to torture the parents so that they couldn't find closure, and to torture Jack with the knowledge that he couldn't give them that closure. While he is sentenced to death, the prospect doesn't frighten him at all, and it's unknown if the sentence was ever carried out.
Victor Paul Gitano from "Fault" was a sadistic pedophile who lures kids in, then rapes, tortures, and kills them. He doesn't discriminate by gender either. On top of that, when trying to hunt Gitano down, things don't quite go as planned. Not only does he kill one of the two kids he had with him in a subway station full of people by slashing his throat, he almost killed Benson the same way. And then, when they corner Gitano in a warehouse, he manages to get behind Stabler and hold him at gunpoint with a shotgun. Gitano, despite knowing he'd get caught or killed at this point, outright boasts that he killed the other child he had (who was a girl) and bragged that she was a "slut" and a "real little whore" before killing her, even though he never got the chance to molest her.
Charlie Baker from "Dominance" has murdered eleven people in about a week, after forcing them to have sex with each other first or raping the victims himself. Then he kidnapped two women, killed their boyfriends, and kept them captive on a roof. He's also been raping his little brother and beating his alcoholic father all along.
Larry Moore aka The Woodsman from "Signature" would use his Torture Cellar to terrorize and torture his victims through every conceivable means which include hanging, burning, maiming, raping and dry drowning them. He meticulously record the sessions, and constantly plays the tapes on a loop to torment current and future captives, and occasionally abduct more than one woman at once so he could have a captive audience and so one victim can listen to another being tortured.
William Harris from "Behave". He rapes Jennifer Love Hewitt's character, tracked her down whenever she was on the verge of putting her life together.....and repeating the process again and again. And he did it with other women as well. Fortunately, the team nails him on kidnapping, ensuring that he will spend the rest of his life behind bars.
Henry Messner from "Born Psychopath." He has his parents wrapped around his finger for a while, he abuses his sister, he lies about everything, he ties her to a bed while trying to set the house on fire to see if she could "melt from the inside out," he shot a cop, he slashes his mother's hand, he locks a kid in a closet, threatens to kill another one, blames everyone else for his own transgressions, and KILLS HIS NEIGHBOR'S DOG. He has no emotions. No empathy at all. What makes him special? He's ten years old.
The guy from the episode based on the Jon Benet Ramsey murder. He was a rich news reporter who kidnapped the Jon Benet Expy to get a ransom from her mom, killed her for fear that she might identify him by smashing her head into a washing machine door, and then making his mentally stunted son think he did it.