Deadly Strength (3.5e Flaw)
From D&D Wiki
It is suggested, young warrior, to just keep your hands in your pockets and don't touch anything.
No matter what your character's intentions are, your attacks always cause lethal damage... even with your fists.
Effect: You may not chose to deal nonlethal damage. Attacks you make with weapons that cause nonlethal damage, including unarmed strikes, must be made to deal lethal damage instead. If you touch a creature, willing or not, you must also make an unarmed attack roll against them.