Gemara

(Rabah bar Rav Huna, also Tana d'vei R. Yishmael): Beis Din may not appoint a son to lash or curse (excommunicate) his father for any Aveirah other than a Mesis, about whom it says "V'Lo Sachmol v'Lo Sechaseh Alav".

Sanhedrin 85a - Question: May Beis Din appoint a son to be their Shali'ach to lash or curse his father?

Answer (Rav Sheshes): A Shali'ach of Beis Din may lash or curse because honoring Shamayim (by punishing sinners) overrides the Isur to hit or curse a Yisrael. Likewise, it overrides the Isur to hit or curse a parent!

(Beraisa): If Reuven was being taken to be executed, and his son hit or cursed him, he is liable. If someone else hit or cursed him, he is exempt.

Question: What is the difference between the son and a stranger?

Answer #1 (Rav Chisda): The case is, Reuven was refusing to go. (Beis Din may make a stranger a Shali'ach to hit or curse him, but not his son.)

Answer #2 (for Rav Sheshes): Reuven was not refusing. "V'Nasi b'Amcha Lo Sa'or" - one may not curse one who acts like your nation. We equate the laws of hitting and cursing.

Question: If so, also the son should also be exempt!

Answer: Rav Pinchas answered (elsewhere) that he did Teshuvah.

Yevamos 22b (Mishnah): (Even a Mamzer) is liable for striking (or cursing his father).

Question: The father of a Mamzer is a Rasha. He is not Amcha!

Answer: Rav Pinchas answered (elsewhere) that he did Teshuvah. Even though "the crooked cannot be fixed" applies to one who had Bi'ah with Ervah and fathered a Mamzer, now he acts like your nation.

Rishonim

Rif and Rosh (Yevamos 5a and 2:3): We must say that he did Teshuvah only to explain why the son is liable. He may not hit or curse him even if he did not do Teshuvah.

Rif, Rosh (Sanhedrin 19a and 10:1) and Rambam (Hilchos Mamrim 5:14): A son may not be appointed to lash or curse his father for any Aveirah, except for a Mesis.

Question: R. Yerucham says if the father did not repent, his son can be a Shali'ach to hit or curse him. This is unlike the Gemara and all Poskim! R. Yerucham himself said above that he is liable for hitting or cursing his father, and Beis Din cannot make him a Shali'ach of Beis Din to do so!

Answer (Shiltei ha'Giborim 1): R. Yerucham holds that the answer 'the Beraisa discusses when he repented' was proper. This shows that the question (can a son be a Shali'ach Beis Din) was when the father did Teshuvah. Rav Sheshes holds that if he did not repent, a son is exempt in any case. Surely he may be a Shali'ach Beis Din! Tosfos in Yevamos says similarly. The Mordechai explains according to the Rift that if the father did not repent, his son is not killed for hitting or cursing his father. We can resolve R. Yerucham. If one fathered a Mamzer, as long as the son is alive, this recalls his sin: The father's Teshuvah does not fix things. However, (now) he acts like your nation. The Gemara says that he is liable for hitting or cursing his father, i.e. if he was not a Shali'ach Beis Din. R. Yerucham teaches that he is exempt if he was a Shali'ach Beis Din. All agree to this.

Rambam (Hilchos Mamrim 5:12): If one's parent was a total Rasha, even if he (or she) was sentenced to death and going to be executed, he may not hit or curse him. If he did, he is exempt. If he repented, the son is liable and is killed, even though his parent is going to be killed. Anyone else who hits or curses him is exempt, even if he did Teshuvah, since he is going to be killed.

Kesef Mishneh: A Beraisa obligates a son who hit or cursed his father being taken to be executed. If someone else hit or cursed him, he is exempt. We say that the case is that the father did Teshuvah. This implies that had he not done Teshuvah, even the son would be exempt.

Rambam (13): If Beis Din appointed Reuven to give lashes, and his parent transgressed and must be lashed, he does not give the lashes. Similarly, he may not be the Shali'ach Beis Din to excommunicate his parents. He should not push or disgrace them, even if they deserve this and did not repent.

Radvaz: Excommunicating is included in cursing.

Question (Lechem Mishneh): According to the Rambam, Rav Sheshes could have established the Beraisa to discuss when the father did not do Teshuvah, for even then the son cannot be a Shali'ach to hit him!

Answer (Lechem Mishneh): According to the Rambam, at first we thought that the father did not do Teshuvah. We had to say that he does not refuse to go. The conclusion is like Rav Sheshes, that the Beraisa is whether or not he refuses, but we must say that the father did Teshuvah.

Question (Tosfos Yevamos 22b DH keshe'Osah): Sanhedrin 85b says that Beis Din may not appoint a son to lash or curse his father, except for Mesis. This is when the father did Teshuvah. If so, no one may curse him!

Answer #1 (Tosfos): If the father said Hash-m's name l'Vatalah or was worthy of Niduy, he is not exempted through Teshuvah.

Answer #2 (Lechem Mishneh): The Rambam forbids a son to hit or curse his father, even if he did not repent, so Beis Din may not appoint him for this.

Poskim

Shulchan Aruch (YD 240:18): A Mamzer is obligated to honor and fear his father. Even if his father is a Rasha, he must honor and fear his father.

Rema: Some say that he need not honor a father who is a Rasha unless he did Teshuvah.

Ha'Makneh (Kidushin 32a DH Mani'ach): Why did the Gemara need a verse to teach that a son may not heed his father to transgress? If a son could obey, Yesh Shali'ach l'Devar Aveirah, so the father would be a Rasha! Also, in any case the Meshale'ach is liable b'Yedei Shamayim! We must say that the father was Shogeg.

Pischei Teshuvah (15): The Pnei Yehoshua (Bava Kama 94b Tosfos DH bi'Ymei) says that this is if he sinned repeatedly, but not if he sinned only once. This answers ha'Makneh's question.

Note: If so, why did the Gemara suggest that a Mamzer need not honor his father? Perhaps his father sinned only once! Perhaps we must say that his parents lived together for a while, for if not, we would not assume that this is his father (since he had no Kidushin with the mother. Perhaps she was Mezanah and became pregnant from someone else!) In some cases, surely a Mamzer must honor his father, e.g. his father is a Mamzer (and never sinned). Perhaps the same applies if his father sinned only once!

Shulchan Aruch (YD 241:4): If one's parents were total Resha'im, even if they were sentenced to death and were going to their execution, he may not hit or curse them. If he did, he is exempt. If they repented, he is liable and he is killed, even if they were going to their execution.

Gra (6): This is like the Rif. Tosfos holds that if he did not do Teshuvah, he is not Amcha and there is no Isur to curse him. The Beraisa must discuss one who did Teshuvah.

Birkei Yosef (4): Rishon l'Tziyon (96) says that the Shulchan Aruch here and the Rambam (5:12) say total Resha'im to teach that one may not hit them, but he does not honor them. When they discuss the Chiyuv to honor a parent who is a Rasha (240:18 and 6:11), they say just 'Rasha'. This Diyuk is not necessarily true. Bereishis Rabah (39:7) says that Hash-m told Avraham 'I exempt you from Kivud Av va'Em, but others are not exempt.'

Note: This implies that Kivud Av va'Em applies even to a total Rasha (Terach was an idolater). However, perhaps Hash-m needed to exempt him because Terach repented (see Rashi Bereishis 15:15).

Shiyurei Berachah (1): The Zohar says that Rachel died and was unable to raise Binyamin because she pained her father Lavan, even though she took the idols in order to stop him from doing idolatry.

Chasam Sofer: If the parent repented but resists going to his execution, the son cannot be a Shali'ach of Beis Din. For this law, regret is Teshuvah, even though he does not accept the ruling of Beis Din.

Shulchan Aruch (5): If Reuven was appointed to give lashes in Beis Din, and his parent transgressed and must be lashed, he should not give the lashes. Similarly, he should not be the Shali'ach of Beis Din to excommunicate his parents, not pursue or hit them due to Shlichus of Beis Din, even if they deserve this and did not repent.