Jad Karadin was a retired witcher of the Cat School. He went by the alias Roland Treugger and was considered to be a respectable trader and philanthropist in Novigrad, where he lived with his wife and two adopted children.
Karadin formerly led a band of assassins, of which Selyse, Lund, Hammond and Vienne were members. According to Karadin, the Cat witcher Aiden accepted a contract to lift a curse off a duke's daughter, took the coin, bungled the job, and then left once the girl passed on. His group was hired as debt collectors, but when asking Aiden for his swords to repay the money he'd spent, tempers flared. Jad blamed Aiden's death on Vienne, who lost her nerve in her position as a sniper, hitting Aiden square in the eye.
At some point after Aiden's murder, Karadin's band broke up, though he never severed ties with Hammond—his right-hand man—the two going into business together. Karadin owned the ship "the Pearl of the Coast", which ran between Novigrad and Skellige, and traded in live goods. He changed his name to Roland Treugger, and despite his shady career was officially a respected Novigrad trader and philanthropist. He listed Miss Marabella's institute and Novigrads' Widows' and Orphans' Relief fund amongst the places he gave small donations to.
Karadin took in the woman Laetitia, embracing her two children as his own. Laetitia was said to know all his secrets and prayed for him each day, which Jad believed to be working.
In 1272, after finding several members of Karadin's gang and tracking them down, the witchers Geralt of Rivia and Lambert, a friend of Aiden's, found Karadin in his estate in Novigrad. The retired witcher explained his side of Aiden's slaughter, and Karadin's fate was left to Geralt.
If Geralt decides to kill him: Geralt told Lambert to do what he wanted, it's his friend therefore his vengeance. Whether Geralt backed Lambert up in this fight or not, Lambert will kill Karadin himself.
If Geralt decides to spare him: Geralt told Lambert to leave Karadin. He told Lambert to recognize he's too angry to think rationally and should trust Geralt's calmness instead. Karadin thanked Geralt for sparing him, while Lambert told Geralt he won't forgive him. Geralt explained to Lambert that he didn't feel Karadin deserved to be murdered in cold blood, suggesting Lambert can go back on his own otherwise. Lambert replied he feels like beating Geralt up, and with that walked away.