What do we learn from bi'Nfol Oyvecha Al Tismach"?
Megilah 16a: That applies to an enemy who is a Yisrael. Regarding others 1 it says "and you will step on their high places."
R. Yonah: This is put next to "u'Rsha'im Yikashelu v'Ra'ah." It warns that when the Rasha - your enemy - falls, do not rejoice. This warns his fellow Rasha, who does not rejoice over his fall l'Shem Shamayim. He does not hate him due to his evil, only because he is his enemy.
16a: Haman bent down, so Mordechai could step on him to mount the horse. As he did so, he kicked him. The Gemara implies that one may rejoice at the fall of Nochrim (PF). Ben Yehoyada - this is only for Amalek.
Why does it say "uvi'Kashlo Al Yagel Libecha"?
R. Yonah: Simchah is greater than Gilah, therefore it mentions Simchah regarding his fall, and Gilah regarding his stumbling.
Malbim: Simchah is constant, for something constant. Gil is for a new matter, e.g. good tidings. Do not have constant joy that your enemy fell from his level. And even when he stumbles - the beginning of falling - do not rejoice at the time.
