You should protect those around you. In some way, every living being is associated with another. I think they have a term for it...

[He pauses, and wipes away some blood on his face distractedly. When he brings his hand from his face, the cut and the blood are gone.]

Six degrees of separation? In this way, the whole universe is your family.

And if it had been your brother, being attacked? What then? Maybe that was somebody's sister, who would have needed you to protect theirs on their behalf.