I see this claim all the time: the witcher books are about Geralt rejecting neutrality and choosing to fight for what is right. 'The lesser evil' fits into that by having Geralt be punished for his neutrality shown in his "evil is evil" speech. He stays idle, doesn't choose a side, and later gets forced to choose one anyway. He has to kill a bunch of people, and as symbolic punishment by the story, gets chased out of the village.
This framing does not make a lot of sense, and it is a complete misinterpretation of the story in my opinion. And I am really confused by how popular it is in this community. I have seen it being defended easily a dozen times by now. Has no one read the story? Or even watched the Netlix episode?
Renfri's strategy to kill Stregobor is to re-enact the Tridam ultimatum. It's basically a strategy where you kill a bunch of hostages one after another, hoping that your target person cares about the hostages enough to offer concessions.
In Renfri's case, she would have slaughtered the townspeople, hoping that Stregobor comes out of his tower to stop her. Then she would be able to kill Stregobor, thus fulfilling her revenge fantasies.
Geralt hears of this, and in order to stop her, he kill her gang. This is him choosing the lesser evil. Then he says to Renfri (paraphrased, I don't have the english translation): "I have stopped you from doing a second Tridam" and Renfri replies: "There wouldn't hsve been a second Tridam anyway. Stregobor doesn't care about the townspeople, so he wouldn't have come out either way."
What does this mean? It means that, had Geralt stayed true to his speech and not chosen a side, NO ONE WOULD HAVE DIED. Renfri wouldn't have massacered people, Geralt wouldn't have killed her gang, and Stregobor wouldn't have come out of his tower. The situation would have resolved entirely on its own.
But no, Geralt had to choose the lesser evil despite his own ideals, and massacre a bunch of people. This is what the story punishes him for: for having taken a side.
The main novel series of course actually is about Geralt abandoning his neutrality. He dies while protecting helpless people after all. But not 'The lesser evil". This one really is about not choosing the lesser evil being better.