Marcellus' is not a story of pacifism, but the story of a man who recanted his soldierly duty to do homage to the idols of Rome. This was done knowing that the result was death. Throwing down his standard and declaring his devotion to Christ before his fellow soldiers broke his devotion to them in their minds. Imagine how they felt - his brothers in arms; he declared that he would no longer have their backs; devotion to the gods was devotion to them; he was a deserter! Deserters died.
But the truth is that Marcellus transforms for us what it meant/means to be a soldier. It was not that he would not be devoted to them as a brother, have their backs, die with them if necessary for a just cause but that the devotion required to do those things did not just have to go to the Roman gods.
Sometimes the peace of Christ requires of us to fight. Sometimes it is to fight for justice, sometimes it is to fight tyranny and oppression. Sometimes we fight with our words, money, talents; sometimes we fight with our lives. No greater love than this exists: that we should lay our life down for our friends. Marcellus does this - to further the salvation of the world and those he loved like brothers.
The natural rights with which We have been dealing are, however, inseparably connected, in the very person who is their subject, with just as many respective duties; and rights as well as duties find their source, their sustenance and their inviolability in the natural law which grants or enjoins them…. Once this is admitted, it also follows that in human society to one man’s right there corresponds a duty in all other persons: the duty, namely, of acknowledging and respecting the right in question. For every fundamental human right draws its indestructible moral force from the natural law, which in granting it imposes a corresponding obligation. Those, therefore, who claim their own rights, yet altogether forget or neglect to carry out their respective duties, are people who build with one hand and destroy with the other.
-- Pacem in Terris, 28-30, John XXIII