In warfare, your goal is to defeat the enemy. Yes, you're willing to kill the foe to achieve that end; but incapacitating the foe, inducing him to surrender, or routing him into headlong retreat are all victories, too. In a D&D encounter, all of those get rewarded with XP for defeating the challenge.
In the Boon of Souls, if the opponent doesn't die, he presumably doesn't qualify as having gone to his afterlife reward. So we have a mechanic here that involves a direct reward to a PC for cutting the throat of a foe who has surrendered.