U.S. GI's in 1945 were told to kill ON-SIGHT any S.S. officer they came across. No prisoners. DEAD. ISIS fighters are no better than SS. You can be sure every one of them has murdered women and children before.
The Geneva Convention agreement of 1929 only covered treatment of wounded military, not healthy combatants. So technically, this would be allowed. To add to your statement about SS officers, any German soldier belonging to a named division (Grossdeutchland, Leibstandarte SS Adolph Hitler, Goering, etc.) was also susceptible to immediate execution.