Leo Bonhart was a bounty hunter originally tasked with killing or capturing a young girl named Ciri on behalf of two different patrons. However, when he found her, instead of doing either, Bonhart decided to take Ciri for himself. Before that though, Bonhart enjoyed himself by slaughtering the Rats, a criminal group Ciri was staying with who robbed from the rich and gave to the poor. Murdering them and cutting their heads off, Bonhart saved Ciri's lover, Mistle, for last, forcing Ciri to watch as Mistle died slowly and painfully before he cut off her head as well. Once Ciri was in his clutches, Bonhart beat her, drugged her with fisstech, and forced her to kill people in fights to the death at the Claremont arena. Bonhart would later kill Neratin Ceka when he tried to help Ciri escape from Bonhart, Rience and Skellen at the village of Unicorn. Deciding to hunt Ciri down for Vilgefortz, Bonhart first asked if he could watch Vilgefortz's planned vivisection of Ciri as part of his payment. Bonhart eventually participated in the defense of Stygga Castle, where he once again attempted to kill Ciri and succeeded in killing her ally, Cahir. Bonhart relished fighting and killing more than anything in the world, attempted to rape a prisoner at one point, expressed a desire to rape Ciri either before or after impaling her on his sword and feeling her slowly die, and was so evil that a psychic found the sensation of scanning his mind akin to sticking her head into a freshly opened grave.
Azar Javed, the leader of Salamandra, leads the despicable organization that uses rape, murder, torture and treason to further their agenda, even kidnap children to experiment on them, mutating them and sometimes even just discarding them if they die during experiments (such as Raymond's son). He also murders Raymond and takes his place in order to get to Geralt, tricks Shani into giving the prisoner a poisoned drug and turns the mortally wounded White Rayla into a freakish mutant and sics her on Geralt.
The Reverend, the spiritual leader of Vizima's Outskirts seems to be a fair, if slightly rude priest at first glance, but is actually a Knight Templar psychopath. He has his own pregnant daughter thrown out of the village for refusing to follow the psycho-cult religion he does, forcing her to become a prostitute just to get by, and tricks his own people into believing that the innocent, kindly mage Abigail is responsible for the recent rise in monster attacks, forming them into a howling lynch mob. He does this merely out of petty spite and prejudice. If Geralt (rightfully) prevents the lynching, even after saving the village from the monsters, the Reverend will try to have him killed, again purely out of petty spite. He doesn't fight Geralt himself first, but manipulates dozens of innocent villagers to attack, willing to sacrifice the lives of his own village just because Geralt hasn't let him murder an innocent woman.
In the swamp region there is a minor character known only as "Gramps" who is a Serial Killer and cannibal who preys on the swamp folk. He seems harmless at first and gives Geralt a fair amount of information and offers him a few useful items. It's not until you visit his house that you discover his secret, at which point you can choose to fight him or let him go.
Bernard Loredo from the second game, who can be best described as a greedy and selfish bastard with zero sense of empathy. The description is spot-on, given that the guy imprisoned, beat and raped an elven woman, who was so broken by these tortures that she killed herself as soon as she gave birth. It also turns out that he was willing to sell Flotsam, the town he was dictating over, to the kingdom of Kaedwen, an act which would render him a traitor to the country.
Whoreson Junior is a figure who runs the city of Novigrad's brothels, gambling, and numerous other businesses. One of these later ones is a brutal gladiator arena where individuals are thrown to be torn to shreds by exotic animals and monsters. He's also a Serial Killer who kills dozens of prostitutes to bathe in their blood. Oh and he hurt Dudu.
The Crones are monsters with the capital M sense as they regularly eat babies, use Exact Words contracts to manipulate locals, inflict devastating curses on individuals, and force the locals to send them regular human sacrifices of their own children. They are based on the Baba Yaga myth and Deconstruct it to show them as nothing more than the horrifying parasites they are.