Also, we all know that guilds are organisations based around groups of people in a profession (e.g. blacksmiths), and to say that the witcher caste in and of itself is a guild isn't necessarily wrong. I think of the different schools as sort of 'regions' that make up this guild. In the case of blacksmiths, different places produce different quality items (Skellige swords will differ from those made in Mahakam). Just like the smiths, witcher schools (such as the Wolf and Viper) produce different witchers, but they all are made in order to slay monsters for coin on the path. What makes witchers interesting is the fact that no one knows really anything about them. They are highly skilled, morally ambiguous, and a certain one we know entertains kings and ploughs sorceresses.
The Cat school stopped producing witchers who walked the path and became assassins, kind of like Serrit, Auckes, and Letho (who originated from the school of the Viper). I imagine that their reputation as assassins who hunt men instead of monsters gives all the other witchers a bad name.
