What do we learn from [bi'Nfol Oyvecha Al Tismach...;] Pen Yir'eh Hashem... v'Heshiv me'Alav Apo"?
Brachos 55b: When Rava fell sick... after the first day he would reveal it... whoever hates me will rejoice... and Hashem will turn His anger away from me.
R. Yonah #1: The Rasha will be punished for rejoicing over the downfall of his fellow Rasha - "u'Fakadti Es Demei Yizre'el Al Beis Yehu" (Hoshe'a 1:4). This is a second reason not to rejoice at his fall - your Simchah will be a reason for Hashem to remove His anger from him!
R. Yonah #2: After warning the Rasha "Al Te'erov Rasha li'Nve Tzadik" (15), it warns like Tzadik not to rejoice over the fall of his enemy, the Rasha 1 , amidst hatred and the quarrel between them. However, one may rejoice l'Shem Shamayim over a Rasha's fall - "Ken Yovedu Kol Oyevecha Hashem" (Shoftim 5:31), "uva'Avod Resha'im Rinah" (above, 11:10).
Malbim: Since Hashem will see the cruelty and Midah of vengeance in you, He will compare his deeds to yours, and your deeds will be bad in His eyes; your enemy will be a Tzadik compared to you, and Hashem will turn His anger from him onto you!
R. Yonah: The verse cannot refer to the fall of his enemy, a Tzadik, for the reason "lest Hashem remove His anger from him", shows that it discusses a Rasha (at whom Hashem was angry), whom one may hate.
