Geralt often makes reference to a personal code for his profession, built around not killing intelligent creatures for money:
'I kill monsters for money. Beasts which endanger people. Horrors conjured up by spells and sorceries cast by the likes of you. Not people.'
... your devil is a sylvan. An exceptionally rare but intelligent creature. I won't kill him, my code forbids it.
It’s your job to kill monsters, isn’t it?’
‘Monsters,’ Geralt said coldly, ‘but not the members of intelligent races.’
What should I say about you, who rejects a lucrative proposition every other day? You won’t kill hirikkas, because they’re an endangered species, or mecopterans, because they’re harmless, or night spirits, because they’re sweet, or dragons, because your code forbids it.
And not involving himself in dilemmas where he sees both sides as morally dubious:
The Lesser Evil
However his principles evolve as he ages, and in private he admits the concept of a code is more of a ploy to assuage people's fears, and he in fact considers his own personal principles more strongly. He is "human", and often faces internal conflict:
'Mistakes? Of course I've made them. But I keep to my principles. No, not the code. Although I have at times hidden behind a code. People like that. Those who follow a code are often respected and held in high esteem. But no one's ever compiled a witcher's code. I invented mine. Just like that. And keep to it. Always—
'There have been situations where it seemed there wasn't any room for doubt. When I should say to myself "What do I care? It's nothing to do with me, I'm a witcher". When I should listen to the voice of reason. To listen to my instinct, even if it's fear, if not to what my experience dictates.
'I should have listened to the voice of reason that time . . .
'I thought I was choosing the lesser evil. I chose the lesser evil. Lesser evil! I'm Geralt! Witcher . . . I'm the Butcher of Blaviken—
That his code is not one shared by all Witchers is alluded to again:
I’ve heard that it has recently become tiresome to negotiate with you witchers. The thing is that, whenever a witcher is shown a monster to be killed, the witcher, rather than take his sword and slaughter it, begins to ponder whether it is right, whether it is transgressing the limits of what is possible, whether it is not contrary to the code and whether the monster really is a monster, as though it wasn’t clear at first glance. It seems to me that you are simply doing too well. In my day, witchers didn’t have two pennies to rub together, just two stinking boots. They didn’t question, they slaughtered what they were ordered to, whether it was a werewolf, a dragon or a tax collector. All that counted was a clean cut.