A squad of soldiers are patrolling Iraq / Afghanistan / Somalia / other lawless hellhole of your choice. From in front of them a person with a assault rifle suddenly emerges from a building and sprays a whole clip into the squad. Some miss, armour stops some but this is 30 bullets at close range - some soldiers suffer injury / death.

He then drops the now empty gun and legs it into the house and out the back.

The soldiers now catch up with who they think did it - this happened in a few seconds after all:

1) Execute him as an illegal combatant - without a uniform the Geneva Convention does not apply. He is an irregular or spy. Is there even now a war between two sovereign states? If not then again the entire framework isn't applicable.
2) Take him as a POW. Although he does not appear to be in an army, so this doesn't really apply - better whip up a "competent tribunal"
3) Arrest him. Although under what laws? Soldiers rarely have powers of arrest. Evidence is finger prints if they're lucky and even if it can be proved he recently fired a gun there are many who might have done so; will the local police even be able to protect his Human Rights?

So... how much effort to get the first person through the system? Which part of the system should even be used? No wonder that in such a case the soldiers probably just shoot him and be done with it.