Status of soldiers taken by illegal combatants?

So there is a discussion I am having about a scene from an anime. A group of armed civilians deemed to be illegal combatants during a war takes an enemy soldier captive. What is his status and what are they required to do with him?

Later, after calls from other soldiers for his release, he attempts to flee and the illegal combatants shoot him. Would this have been considered a warcrime depending on the answers above?