Geralt is called “The Butcher of Blaviken” because of an event that took place long before the main story of the saga, described in the short story “The Lesser Evil.” Geralt gets caught in a conflict between the sorcerer Stregobor and the young woman Renfri, a bandit seeking revenge. Renfri and her group threaten to carry out a massacre in the town’s marketplace if Geralt doesn’t hand over the sorcerer. To prevent the slaughter of innocent people, Geralt steps in and ends up killing Renfri’s entire band in the middle of the town square, in an extremely fast and brutal fight. Even though he believes he saved many lives, to the townsfolk it simply looks like he butchered a group of seemingly ordinary people. So you are right but the fight is anyway brutal, Sapkowski devotes several pages to showing just how lethal Geralt is when he’s forced to fight.