As the great american philosoper, George S. Patton, once said, "the purpose of war is not to die for your country: the purpose of war is to make the other dumb SOB die for HIS country". The Rabbi is quite correct, although not particularly profound, when he says that no mercy should be shown "when no mercy should be shown". Actually, what he said is not only correct, it is a tautology.
Austrian chancellor: Austria and Germany will allow migrants entry (Reuters)
from the article: IDF Chief Rabbi: Troops who show mercy to enemy will be 'damned'