That is a very tricky question, one that was the particular struggle of one Alvin York.
Before Alvin York became a Christian, he lived a very reckless existence, drinking, gambling, and the like. One night, he and some friends, one Everett by name, were engaging a couple of strangers in a card game. The was some disagreement about the legitimacy of some winnings and a fight broke out. In the confusion, Everett was killed.
This shocked Alvin York into his right mind. He saw where his lifestyle led and surrendered his soul to God. He became an ardent Christian and started on a life of responsibility. This went on until the first world war began.
Alvin was drafted into the military, and there was perhaps never a more reluctant soldier. He tried conscientious objection, but was rejected as his church was not part of a recognized denomination. Therefore, he had to go.
His commanding officer was baffled by Private York. Like all children of rural Tennessee, he could shoot the tail feathers off a bird in flight. However, he still didn't want to fight and kill. The officer did something unprecedented and gave Alvin time off to think about it.
Alvin York did think about it, and prayed about it too. He went up to a mountain near his home where he felt close to God and read his Bible. He struggled mightily with the question, sometimes coming to one conclusion and sometimes to the other. He eventually came across this verse.
Blessed are the peacemakers, for they will be called children of God.
Alvin York began to see his problem in a new light. He began to see that sometimes the only path to lasting peace is through violence. He thought of his family and realized that he would not hesitate to fight, or even kill, a man threatening to harm them. Needless to say, he went back to the war and was one of the most honored soldiers in history.
We should always seek the peaceful option, but sometimes there isn't one. There always have, and always will be, those who know no reason. We can restrain them, but not forever. We can pray for their salvation, but it may be refused. Sometimes the only option is to escort them to the Divine Court to be judged for their sins.