Taken from this site:
St. Cyril the Enlightener of the Slavs was approached one day by some Christians who were facing opposition from Islamic militants. They mentioned the passage from Matthew about turning the other cheek, and wanted to know if such a statement from our Lord prevented any Christian of a good conscience from serving in the military. His enlightened response is useful:
St. Cyril said: “If two commandments were written in one law and given to men for fulfilling, which man would be a better follower of the law: The one who fulfilled one commandment or the one who fulfilled both?’
The Saracens replied: “Undoubtedly, he who fulfills both commandments.”
St. Cyril continued: “Christ our God commands us to pray to God for those who persecute us and even do good to them, but He also said to us, ‘Greater love hath no man than this, that a man lay down his life for his friends’ (John 15:13). That is why we bear the insults that our enemies cast at us individually and why we pray to God for them. However, as a society, we defend one another and lay down our lives, so that the enemy would not enslave our brethren, would not enslave their souls with their bodies, and would not destroy them in both body and soul.” (From the Prologue of Ochrid)
Second, Cyril, like most non-pacifists, views pragmatism as defined by humanity too highly. Cyril might argue that if the Saracens didn't take up arms to defend their neighbors, then what good would they do? But as a Christian, my job is not to determine the ends God brings about, but rather to use the means he has prescribed for me. If violence is wrong - if repaying evil for evil is wrong - then those means are off limits for me as a Christian. I can't redefine morality. Perhaps killing an aggressor is less evil than being an aggressor (if you can somehow kill the aggressor without hatred), but it's still an evil. Let me give an example that I think most Christians would agree to, and then apply that to the pacifistic position.
One of the books we read for family devotions has a story in it about a moral choice. The bread winner of a family is asked to do something immoral at work (fudging numbers or something like that). His boss tells him that if he refuses to do it, he'll be fired. The man refuses to participate in the moral action and loses his job. Most Christians would agree that this is the right thing to do, even if they'd have a hard time doing it themselves.
But now what if this family was living paycheck to paycheck. One lost day of work means a day without meals, and more than one day missed means going without utilities, being able to pay rent, etc. They have no family to turn to and their church is filled with impoverished people who cannot help as well. It is the job of the parents to provide for their family. Does the family's dire situation mean that the father should choose to fudge the numbers and keep his job so he can provide for his family? What about if the immoral action was to set up a co-worker so the employer had just-cause to fire them, and then that individual would not be able to provide for their family. What about single mothers with no skillset and no ability to get any job but the job of a prostitute? Not at all. None of these things are justified, and you only have to walk the hypothetial moral compromise down the line until you get to something someone considers so immoral, they'd have to cave. We know that immoral means are not excused on account of our notions of pragmatism. Now we might be able to empathize with someone who chose to do the wrong thing and keep their job. We might be able to feel sympathy for the impoverished single mom who is a prostitute. It's a tough situation to be in. But as Christians we still have to call these acts immoral. We must fiercely love those who choose immorality, just as we love our neighbors and our enemies, but we must not take on evil ourselves in order to accomplish what we determine to be God's ends.
Doing violence to another is the same sort of thing as the scenario above. Just as we aren't to be immoral in our workplace, we are not to do violence to image bearers of God. That is not a moral means we Christians have at our disposal. The situation may be very dire. I may be able to empathize with someone who does violence in certain situations. I may even feel joy when I see violence done to someone like a Hitler or a Stalin. I may one day use violence myself against another human being because I am too weak to submit to the means of God. We can empathize all we want with the use of violence for self-defense, just as we can empathize with a parent who chooses immoral means to provide for their family. But difficulty in implementing the moral means doesn't legitimize or moralize choosing the immoral means.