Why does it say "Al Tere"?
Rashi: You should not have stood opposite. (Rather, you should have helped Yisrael!)
Radak: You should not have seen the day of affliction of your brother. Malbim - due to the Bris of brothers, you should have refrained from seeing.
What is "b'Yom Nochro"?
Rashi: The day they handed them over to the enemy, i.e. the Bavliyim. And so it says in Shmuel I, 23:7 "Nikar Oso Elokim b'Yadi" - He handed him over to me.
Radak: This explains that "b'Yom Achicha" is "b'Yom Nochro." Nochro means that he was estranged from his land and became like a Nochri, for they took him out of [his land].
Why does it say "v'Al Tagdel Picha"?
Radak: Do not aggrandize your mouth to speak in jest or other matters. Also Yonason translates 'because you spoke many great (haughty) matters.'
Malbim: [You should not have rejoiced, and all the more so] not verbalized your Simchah to speak mockery and disgrace. This refers to Churban Rishon, in which they verbally enlarged the Churban - "ha'Omrim Aru Aru Ad ha'Yesod Bah" (Tehilim 137:7).
