Patriotism, being conscripted, and joining an army because one has no other prospects are not crimes that deserve any punishment, although the end result can be the same fate that countries with capital punishment reserve for only the worst murderers.
That 'other guy' is armed, and will use that weapon to kill 'our guys'.
They have done so, even non combatants, and have celebrated that slaughter of those who weer not soldiers.
As a result, 'our guys' justifiably see that 'other guy' as a threat and will kill them first.
If the 'other guys' don't like that, lay down their weapons, strip off the uniform, and surrender.
Otherwise, they will continue to be perceived as a threat, and it is on, with all the nastiness that implies.
Simple enough.