Tarzan the Untamed by Edgar Rice Burroughs

Tarzan of the Apes was disgusted. He had had the German spy, Bertha Kircher, in his power and had left her unscathed. It is true that he had slain Hauptmann Fritz Schneider, that Underlieutenant von Goss had died at his hands, and that he had otherwise wreaked vengeance upon the men of the German company who had murdered, pillaged, and raped at Tarzan's bungalow in the Waziri country. There was still another officer to be accounted for, but him he could not find. It was Lieutenant Obergatz he still sought, though vainly, for at last he learned that the man had been sent upon some special mission, whether in Africa or back to Europe Tarzan's informant either did not know or would not divulge.

But the fact that he had permitted sentiment to stay his hand when he might so easily have put Bertha Kircher out of the way in the hotel at Wilhelmstal that night rankled in the ape-man's bosom. He was shamed by his weakness, and when he had handed the paper she had given him to the British chief of staff, even though the information it contained permitted the British to frustrate a German flank attack, he was still much dissatisfied with himself. And possibly the root of this dissatisfaction lay in the fact that he realized that were he again to have the same opportunity he would still find it as impossible to slay a woman as it had been in Wilhelmstal that night.