As far as the story is concerned, I think the experience of the sniper and the tragic way in which the story ends makes it clear that the overall theme of this impactful story is that violence is not worth the cost to families, individuals and nations. Certainly, the setting of this story, set during the Irish Civil war, bears this out, as not just the country was divided, but cities, streets and families themselves as each took different sides.
Absolutely, violence is sometimes justified. However, I am not at all sure that violence is justified in the circumstances that surround this story.
To me, violence is always justified when your people or your country is attacked and threatened. In other words, if others are using violence and may destroy your people or your country, then violence is certainly justified.
I am not certain that these conditions applied in Northern Ireland, though. I do not believe that, at the beginning, either the Republicans or the Loyalists were actually threatened with being exterminated. So I am not convinced that the violence portrayed in the story is justified.
Violence is use of physical force to make someone else behave according to your wishes or to simply cause harm to others. The justification for violence is dependent on two factors. First is the validity or justification of the objective sought to be achieved by the violence. Second is the availability of other non-violent means to achieve the same objectives.
Violence can never be justified when the objective to be achieved by it are not justified. Thus though it is fine to kill a sheep so that you can eat its flesh and satisfy your hunger. However, we cannot say the same thing about killing of a wild animal as a sport.
Even when the objectives to be achieved are justified, violence may not be the justified, specially when alternative, non-violent means are available to achieve the same objectives.The biggest disadvantage of using violence as a means of achieving your objectives is that it usually sows seeds for further violence in heart of people affected by the violence. This can start a chain of vendetta like violence, which, in the long term benefits no one. In this way, violence committed just for 'getting even' is definitely not justified.
Violence is also not justified when the harm caused to others by violence is disproportionate to the benefit to the person engaging in the violence. Thus, it is okay to fight a war against an aggressor, but carrying it to extreme limits causing unnecessary death and destruction in retaliation for the acts committed by the aggressor may not be justified.