"Was there a trial? "
You want lawyers and a judge?

This guy was a soldier, apparently not in uniform & he murdered a police officer plus it was in a declared cease-fire.
What needs to be determined?
Was he actually wearing some kind of uniform?
Did he claim to be a soldier and therefore deserve to be treated as an enemy soldier per Geneva Convention?
Or do you think this guy was simply an innocent bystander in the wrong place at wrong time.
If the guy claimed to not be a soldier and claimed not to shoot police officer, and your soldiers claimed that this guy did shoot the police officer; the guy gets a bullet in his skull.
I would think a general in a war zone, does have the authority to pass judgement without a formal trial. I think highest commanding officer "in an area" could legally do this.