El Salvador has begun arresting war criminals from its civil war … starting surprisingly with guerrillas accused of killing wounded US soldiers who had fallen into their hands.

It seems plausible to me that the guerrillas were not in a position to hold prisoners. During World War II, it was considered legitimate (though regrettable) to kill enemy soldiers that had surrendered, if it was unfeasible to take prisoners there and then. What do today's treaties say about this situation?