I recently started reading the books that the Witcher game series is based on after thoroughly enjoying the games, but still haven't found a satisfactory answer for this. I understand that witchers were created to kill the monsters that had spilled over into their world, and had to undergo extensive mutations and training to get there.
My question is, why would they be trained to only kill monsters for money? It seems like a huge amount of effort to take these orphans, mutate them, train them and then send them off to be a mercenary. If the whole purpose of their existence is to kill monsters, then to me it just feels like the monetary requirement is more of a hindrance than anything else.
I understand that they are meant to be neutral, and not get involved with politics which makes sense. However, neutrality has nothing to do with it when it comes to monsters.
Am I misunderstanding the principles? Were they always told to only work if they're being paid, or is that a recent thing, due to a lack of monsters, and the general disdain for their trade?