All the enemies are dead except the boss who says he surrenders, what do you do?

[mehs]

You are ignoring that unless you are specifically statted into it, taking people alive is much harder than subduing them with lethal force.

The core disconnect seems to be a difference, intentional or not, in the interpretations of how the scenario arose. In my interpretation, the reasonable assumption for how it happened is if taking the minions alive were unreasonably difficult and presented undue risk. In your interpretation, the paladin and party slaughtered the minions just 'cuz. While the latter is more common from the perspective from players not caring about morality, the former is much more likely from an in game perspective.