The point of this quote by Geralt in the story is that Geralt is wrong and using it as an excuse to avoid taking action to protect innocent lives. The result of this stance of his is that he butchers several people in a public square and gets driven out of town and forever labeled "The Butcher of Blaviken." Only a couple pages after this 'evil is evil' monologue he says, verbatim, "We have to choose the lesser evil!"