[voice]
A handicap. A mental disability that would prevent someone from making a conscious choice to harm someone that are emotionally attached to. Humans without such inhibitions can break under the right amount of pressure, so the only way in which they can be incapable is if they truly lack the mental or physical ability to consciously make such a choice.
There are those with the power to control the wills of others, and by taking away the free will to make that choice, one can perhaps be come incapable.