The sort of international law to which JeneralJihad refers apply in wars where both sides distinguish their forces from the civilian population and take measures to avoid civilian casualties (that does not mean perfection in prevention!). By disguising themselves as civilians, by hiding among civilians, by slaughtering and raping and enslaving civilians while fighting, ISIS and other Jihadi groups forfeit protections of international law.
But even were Jihad John to have complied with international law, he would still be a valid target of war. Just ask Yamamoto Isoroku.
All in all, good riddance Jihad John! He's enjoying his 42 Helen Thomases, courtesy of President Drone.