The Shooting of Hostages in Retaliation

In 1944 it became known to the Free French Partisan fighting forces that the Germans had executed 80 partisans and planned soon to execute more. The Partisans thus decided they would shoot 80 Germans prisoners who had recently surrendered to them. At this point the Red Cross intervened, won a postponement of the executions, and sought an agreement from the Germans to treat captured partisans as prisoners of war, who may not be shot. The Partisans waited 6 days and the Germans did not reply. The Partisans then shot 80 German prisoners. After these shootings the Nazis executed no more Partisans. Was the shooting of morally justifiable? the 80 German prisoners by the If so, why? If not, why not?