Merciful (3.5e Flaw)
From D&D Wiki
Many times you don't have it in your heart to kill.
Effect: Whenever you or an ally you can see brings an animal, dragon, giant, fey, humanoid, magical beast or monstrous humanoid to below 0 hit points and dying, but not killed, you feel compelled to save that creature's life. You are shaken until you or an ally makes a first aid check to stabilize that creature. If you or an ally willingly kills the creature, or you are otherwise unable to tend to it, you remain shaken for 1 hour.