Renegade witchers are witchers who have either betrayed their brethren or been expelled from a witcher school. This usually happened for a crime such as the killing of an innocent man or some other crime equally unworthy of a witcher. There are no references to renegade witchers in Sapkowski's original works. The term appears in both the first Polish film and television series and in The Witcher 3: Wild Hunt journal, in reference to Letho the Kingslayer.
The Season of Storms informs that at least some of the Cat School members are banned from entering Kaer Morhen and don't have a roof over their heads, which could imply that Cats are technically renegades in the Wolves' eyes.