The Implications of Relativism: So long as the assailant in this case does not believe he has done anything wrong, he has indeed done nothing wrong. You have no basis to complain about his actions, even if he acted with full knowledge that you and your family objected to his actions, and that society views such actions as grossly immoral. Although you might wish to avenge his actions by harming him, if you do so your actions are no more praiseworthy (or blameworthy) than his. You may appeal to the law, but no court would be justified on a moral basis to convict or imprison the assailant. If this is done, the actions of the court are no more praiseworthy (or blameworthy) than those of the assailant. In fact, a legal system which would condemn the actions of the assailant would be no better on moral grounds than one that allowed such actions, or even one that would reward the assailant for his actions by paying him $3 million from public funds.
Cultural Relativism
The Situation: There is a country, Xenophobia, that contains
two distinct cultural groups: the majoritarians and the minoritarians.
The majoritarians make up 90% of the population, the minoritarians the
remaining 10%. There is a long history of suspicion and hatred in
majoritarian culture against the minoritarians. According to majoritarian
cultural history, the majoritarians are a culturally superior race that
were the original inhabitants of Xenophobia, and they therefore see minoritarians
as inferior and usurpers of their native lands. You are a minoritarian.
The fragile truce between the majoritarians and minoritarians disintegrates
over a murder case in which a minoritarian is charged with the assassination
of a prominent majoritarian politician. There is a quick move in
the legislature of Xenophobia to round up all minoritarians and place them
in internment camps and to suspend judicial procedures in all cases where
minoritarians are charged with crimes. The bill is quickly approved,
and you and your family are interned. Your brother was charged with
a robbery some years ago, and because the repressive law against minoritarians
is retroactive, the commander of the camp takes your brother to the firing
range where he is executed by firing squad.
The Implications of Relativism: So long as the actions taken by the majoritarians are consistent with their cultural beliefs, there is nothing wrong with their actions. You have no moral basis upon which to complain about their actions, even if the majoritarians understand that world opinion condemns their actions. You and other minoritarians might wish the world community (e.g., the United Nations) to place economic sanctions against the majoritarians, or even intervene militarily, but if it did, these actions against the majoritarians would be no more praiseworthy (or blameworthy) than those of the majoritarians. Alternatively, you and other minoritarians might believe that you are justified in taking up arms against the majoritarians, if you can manage it. But an armed revolt against the majoritarians would not be just or right in any significant moral sense. Morally speaking, such an action would be no more meritorious than the repressive actions of the majoritarians to which you react.