Brotherhood is something innate in man. To the point that everything, absolutely everything we do, say and think is born according to others and inevitably affects them.
Which shows that we are a whole. As the body is a whole and, what we eat, it feeds everything and, if we poison it, we poison everything.
We only need to realize that all men together constitute an organism and that the good of one is the good of the whole and the evil of one is equally the evil of all.