You can shoot them if they are in combat against you, but that's not considered a crime and it would be illegal to arrest them for it. Soldiers are considered to have immunity for acts of war (except war crimes)
So for example it would be a war crime to punish someone for fighting in an opposing army. You can hold them as a prisoner of war for the duration of the conflict, but its supposed to be a means of keeping them out of a fight and not a punishment per se.
I think the biggest difference is that crimes can generally be punished after the fact. A murderer can be punished whenever they are caught. A soldier can be shot at at the time, but if they decide they are tired of the war and run away to a farm or something, they are now civilians and can no longer be shot at or punished for previously being a soldier (unless they comitted war crimes) even if the war is still raging on.
> Soldiers are considered to have immunity for acts of war (except war crimes)
Late edit: to clarify that is soldiers of an actual country have immunity. Combatants of a non-state group do not have immunity, so can be subject to arrest for merely participating in the conflict.
So for example it would be a war crime to punish someone for fighting in an opposing army. You can hold them as a prisoner of war for the duration of the conflict, but its supposed to be a means of keeping them out of a fight and not a punishment per se.
I think the biggest difference is that crimes can generally be punished after the fact. A murderer can be punished whenever they are caught. A soldier can be shot at at the time, but if they decide they are tired of the war and run away to a farm or something, they are now civilians and can no longer be shot at or punished for previously being a soldier (unless they comitted war crimes) even if the war is still raging on.