Why does it say "Yoser Letz Loke'ach Lo Kalon"?
Orchos Tzadikim (Sha'ar ha'Shetikah DH ha'Shelishi): A Letz is Chacham in his own eyes, therefore he mocks another's actions.
Malbim: The Na'aros of Chachmah call only to the Pesi and Chasar Lev, but not to Letzim and Resha'im. The latter will not hear the call of Chachmah. One who rebukes a Letz takes disgrace for himself.
What is the meaning of "u'Mochi'ach l'Rasha Mumo"?
Rashi: It is a blemish to the one who rebukes him, for the Rasha reviles him and does not heed him. This is a warning; it is forbidden to talk with people who entice from the straight path, even to rebuke them and draw them close.
Malbim: Musar is via fear of punishment. Tochachah is via intellectual debate and proofs. A Letz does not scoff at intellectual proofs, only on the laws of Chachmah, for there is no proof for them in Da'as. Its primary support is Yir'as Hashem - Musar Chachmah. One must receive it because Hashem gave it, and fear Hashem and His punishment - "Yir'as Hashem Reishis Da'as" (1:7). The Letz accepts only what has intellectual proof; he will disgrace one who rebukes him without proof. The Rasha sins b'Mezid; he does not heed even Tochachah of Sechel, even with proofs. He disgraces one who rebukes him with his own blemishes - he will call him a Rasha.
