Of course we should strive to make society just, and this will reduce the motive for violence -- the mistake is to believe that doing so will stop all violence. This might seem a small point, but it's not: Believing that it would stop all violence is a sign that you believe that (a) people are fundamentally good, and (b) have no agency -- that we just react inevitably and helplessly to conditions imposed on us by an external environment. Both of these beliefs are dangerously wrong.