Well here is the Geneva convention take on Mercenaries
Article 47.-Mercenaries
1. A mercenary shall not have the right to be a combatant or a prisoner of war.
2. A mercenary is any person who:
(a) Is specially recruited locally or abroad in order to fight in an armed conflict;
(b) Does, in fact, take a direct part in the hostilities;
(c) Is motivated to take part in the hostilities essentially by the desire for private gain and, in fact, is promised, by or on behalf of a Party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that Party;
(d) Is neither a national of a Party to the conflict nor a resident of territory controlled by a Party to the conflict;
(e) Is not a member of the armed forces of a Party to the conflict; and
(f) Has not been sent by a State which is not a Party to the conflict on official duty as a member of its armed forces.