He was just a plain old evil man.
His ‘mercy’ was such that he knew he could never get away with a wholesale slaughter of Americans, and not have it fly in his face, with the consequences eventually leading to his own neck swinging from the gallows.
America was a firm Christian nation back then. Not even the Yankee vets would have gone along with that 100%. His restraint from doing that is evidence of that culture.
He was a violent psycho.