There are different levels of human violence; physical, psychological, social, economic, sexual, emotional, spiritual, cultural, verbal, symbolic and negligent. Scientists believe that humans are a self-domesticated animal and that this occurred by Stone Age humans systematically killing fellow humans who exhibited reactive aggression. This largely removed the instinct for reactive aggression from the gene pool (still very much present among chimpanzees) but humans retained the capacity for proactive aggression which was necessary for killing the reactive aggressors.
Let us now draw a distinction between justified and unjustified violence. Of course all perpetrators of proactive violence violence believe that their violence is somehow justified. So in order to find out whether the violence is justified must we investigate whether there is a) a better (i.e. less violent) and b) indeed viable alternative. If there is not then the violence is justified. Is Israeli violence against Palestinians since 1947 justified? Generally yes, because the alternative was always genocide against the Jews which is a far worse violence. Are prisons justified? No, because criminal reservations are a much less violation. Is pacifism a viable alternative to war? No, because democratic disarmament leads to military aggression by dictatorships against democracies. Pacifism would only work if all countries were liberal democracies which they are not. Yet we must not be satisfied with the answers that we have, we must innovate better (i.e. less violent) and viable alternatives to current violence in order to replace it.