Morally airtight. But it might not have been purely cowardice on the grandfather's part.
You know how you generally avoid straight up crushing a stinkbug, instead preferring to redirect it outside, because otherwise it will detonate with noxious fumes?
Killing a nigger is similar, except instead of noxious fumes, it detonates with a new version of reality where you are now in jail for the rest of your life. Every White person is viscerally aware of this even if they can't all articulate it. And it's automatically factored into their subconscious risk assessment anytime they are presented with violence against a nigger as a possible solution to a problem, and that violence can't be performed without witnesses.
You know how you generally avoid straight up crushing a stinkbug, instead preferring to redirect it outside, because otherwise it will detonate with noxious fumes?
Killing a nigger is similar, except instead of noxious fumes, it detonates with a new version of reality where you are now in jail for the rest of your life. Every White person is viscerally aware of this even if they can't all articulate it. And it's automatically factored into their subconscious risk assessment anytime they are presented with violence against a nigger as a possible solution to a problem, and that violence can't be performed without witnesses.