Semirhage from the Wheel of Time. She was a torture specialist who would use the one power and her knowledge as a top healer of the body and mind to use both pain and pleasure to turn her victims to the dark or send them to attack their loved ones. She was effective enough that they tended to either succeed or fail because they were found out, rather than the person deciding against it once they were free. Even those who had turned and were restored using age of Legends mind restoration techniques lived in constant fear of ever being in her hands again. They had to take special precautions to prevent prisoners from committing suicide after finding out they were being put in her charge. She was a sadist who would cause unnecessary levels of pain during her healing even before the bore was drilled and joined the dark because she was caught and offered the choice of oath rod vs violence or being severed from the source.

Though some of the other Forsaken give her a run for her money.

Graendal with her mind slaves that she would use for pleasure or just furniture.

Sammael who, when told that he was giving his prisoners of war half the food they needed to survive, immediately ordered half to be executed (and generally just fed them to the trollocs even when they had enough food).

Aginor who joined the shadow because they wouldn't let him do the experimentation on animals and people that he wanted to. He then created the trollocs and other shadowspawn and took 10k people per day for experiments and fodder from the start of the war power to the end of it, potentially doubling that number in the last 5 years of it.

Moghedian who ran evil schools that had children at first informing on their loved ones and later carrying out their executions themselves. During the breaking of the world, these children caused chaos that seemed like they were racing against male channellers going mad to break the world.

Balthamel established breeding camps for humans to be fed to trollocs. He also had a temper that almost got him oath rodded before the war of power.

Plus just general genocide, terror, manipulation, war, and oppression by all 13 of them. These ones stand out a bit more in their cruelty. Ishamael ultimately didn't want to be reborn anymore, Lanfear was obsessed with Lews Therin, Asmodean wanted to live forever and be a well known composer. Demandred, Be'lal, and Rhavin were jealous of Lews Therin's success. Mesaana wanted to be a researcher. They all did evil shit and didn't care about the people they hurt, but the ones in paragraphs above seemed to do it as the ends, rather than a means to an end.