“Things looked very different if you were a participant in civil war.

Although its foundation myth included fratricide, as Romulus killed Remus, and after the internal conflicts which brought down the Republican form of government civil war haunted the Roman imagination, Rome during most of its history was rather better than Greek cities at avoiding civil war, possibly in part because of its relentless militarism towards other powers.

Yet at times civil war was a real as well as ideological threat.

Other civil wars had been bad, but nothing compared to this one. In others the aim was a change of government, here the
intention was the destruction of the state (3.24–5).

Not only had they tried to recruit barbarian Gauls into the plot, but the conspirators themselves had become like barbarians: they were marked by criminal audacity (audacia), impious crime (scelus), and mad rage (furor, 1.31 etc.).

In civil war it was necessary to show that your opponents had given up the right to be treated as fellow citizens; instead, being like barbarians or even worse (3.25), they deserved to be declared enemies (hostes) of the state.

To fight another Roman it helped if you could show that he was not a Roman at all!” (Sidebottom, 2004, p.59-60).

Reference

Sidebottom, H. (2004) Ancient Warfare: A Very Short Introduction. Oxford: Oxford University Press.