If these forces had abandoned their positions where they should have been protecting the city...
If they'd left vulnerabilities in defenses that only hadn't converted into disasters because somehow these nobles were controlling the mutants somehow...
Then they weren't simply an army deserving respect as combatants doing their duty.
They were deserters who'd put the civilian population at risk for political purposes. Leaders had ordered them to abandon fundamental responsibilities. And that created a situation where punishment wasn't simply permitted but obligatory under military laws governing the armed forces' conduct in the kingdom.
The laws were clear, had been clear for centuries. Abandoning a defensive position without authorization was a capital offense.
Not because rulers were cruel but because the alternative was systematic collapse of defense that would result in far more deaths than executing a few deserters.
