According to the rules of engagement, you sometimes do show mercy to the enemy. If a combatant is wounded and on the ground it's illegal to shoot him. It violates the laws of war. The article didn't clarify what the rabbi means by the enemy. Does he simply mean Hamas militants or Palestinians as a whole? Regardless, he is still advocating war crimes. He should be fired for this. The fact that he hasn't been fired for his statements in the past shows Israel is not serious about following international law when it comes to the rules of war. Israel expects the enemy to keep captured soldiers alive, and the enemy is supposed to be kept alive, not killed. Such advice could endanger Israeli soldiers if they followed it with a bad tit for tat.
U.S., allies conduct 31 air strikes in Syria and Iraq against ISIS militants (Reuters)
from the article: IDF Chief Rabbi: Troops who show mercy to enemy will be 'damned'