Leo Bonhart is one of the main villains in the Witcher series. He's a human bounty hunter, assassin and a former soldier.
Bonhart was a professional bounty hunter notorious for his viciousness and cruelty. Among his grisly trophies collected from victims included a Witcher's medallion, and Bonhart indeed claimed that he had in fact killed a Witcher in his day. Though it's not confirmed that he did, it would not be surprising given his high level of fighting skill and ferocity.
At one point, Bonhart was hired by Stefan Skellen to kill Ciri and by Baron Casadei to capture her alive. Whilst pursuing her, he killed a benevolent group of outlaws Ciri had fallen in with known as the Rats who robbed from the rich and gave to the poor. He decapitated as well as killed them, and saved Ciri's love Mistle for last, forcing Ciri to watch him kill Mistle slowly and then take off her head as well.
Once he finally got his hands on Ciri, he turned her into his slave and forced her to kill in the arena at Claremont. Eventually, Ciri escaped him, and for helping her, Bonhart killed Neratin Ceka. Bonhart would then go on to work for Vilgefortz, helping him in his own hostile efforts against Ciri and others and expressing a sadistic desire to watch Vilgefortz perform his fatal experiments on Ciri. Bonhart later rejoined him during the showdown at Castle Stygga. During the battle, Bonhart killed Geralt's companion Cahir, but was then killed by Ciri.
The Witcher 2: Assassin of Kings
The Witcher 3: Wild Hunt