"But one has sinned," said I. "Him I will kill, whenever I may meet him." (21.17)
The one in question is Hyena-Swine, the only Beast Folk Prendick truly fears. Notice how Prendick manipulates the moral system to turn his enemy into an enemy of the moral code itself by deeming him a sinner. Prendick has learned to play the moral game to his advantage, and that's pretty tricky of him, if perhaps not exactly, um, moral.