If your enemy surrenders, take them prisoner. Give them no more reason to fight you, no more reason to hate or fear you. Make your enemy your friend, and you have increased your numbers for your cause. No stronger devotee than one who as seen both sides with unclouded eyes and has made the decision of whom to support by themselves.
If your enemy refuses to surrender, grant them immediate release. Consider yourself before you wound; would you want to live on, to be in agony for however long your life bled out of you? It is true that a wounded foe will drain your enemy's resources faster, but in victory, do you want to be remembered as someone who enforced those tactics?
To those who fight with dishonor, beware; the next uprising may paint you as the enemy.