Gemara

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

Question: The Isur "Do not curse a Nasi in your nation" applies to one who acts like your nation. (Likewise, there is no Isur to curse a wicked father, e.g. one who fathered a Mamzer!)

Answer: We answer like Rav Pinchas said (elsewhere). The case is, he repented.

Question: One cannot repent for this!

(Mishnah - R. Shimon ben Menasiya): "The crooked cannot be fixed" applies to one who had Bi'ah with Ervah and fathered a Mamzer!

Answer: In any case, now he acts like your nation.

Question (against R. Yochanan - Beraisa): If a man died, leaving his orphans money of usury that he collected, even if they know this, they need not return it.

Inference: They need not return it, but their father would have to return it!

Answer: Really, even their father is exempt. The Beraisa discusses the children for parallel structure to the Seifa.

(Seifa): If their father left them a cow, garment or any specific item, they must return it for the honor of their father.

Question: Children need not honor such a father!

"Do not curse a Nasi of your nation" - one who acts like your nation (but we are not concerned for honor of the wicked)!

Answer: We answer like Rav Pinchas answered (elsewhere). The case is, the father repented.

Question: If he repented, he should have returned it!

Answer: He died before he was able to.

Kidushin 32a (Beraisa): If one sees his father transgress a Torah law he should not say 'Father, you transgressed', rather, 'Father, this is what the Torah says.'

Objection: That will pain the father!

Correction: Rather, he says 'Father, there is a verse in the Torah that says...'

Rishonim

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

Rif (Kidushin 13a): If one sees his father transgress a Torah law he should not say 'Father, you transgressed', rather, 'Father, this is what the Torah says.'

Rosh (Kidushin 1:50): He says 'Father, there is a verse in the Torah that says...'

Rif and Rosh (Bava Metzi'a 34a and 5:5): If a man died, leaving his orphans money of usury that he collected, even if they know this, they need not return it. If he left them a specific item, they must return it for the honor of their father only if he repented and died before he was able to return it.

Rambam (Hilchos Mamrim 5:12): If one's parents were utter Resha'im, even if they were sentenced to Misas Beis Din and they are going to their execution, he may not hit or curse them. If he did, he is exempt. If they had done Teshuvah, he is liable and is killed, even though they are going to be killed.

Kesef Mishneh: A Beraisa obligates a son who hit or cursed his father while the father was going to be executed; the case is, the father did Teshuvah. This implies that had he not done Teshuvah, the son would be exempt.

Rambam (13): 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. He should not impel or disgrace them, even if they deserve this and did not repent.

Radvaz: Excommunicating is included in cursing.

Tosfos (Yevamos 22b DH keshe'Osah): The Gemara (Sanhedrin 85b) says that Beis Din may not appoint a son to lash or curse his father for any transgression except for Mesis. This is when the father did Teshuvah. If so, no one may curse (excommunicate) 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 13): According to the Rambam, even if the father did not do Teshuvah the son may not hit or curse him, so Beis Din may not appoint him for this.

Rambam (6:11): A Mamzer is obligated to honor and fear his father, even though he is exempt for hitting or cursing him before the father does Teshuvah. One must honor and fear his father even if his father is a Rasha that transgresses.

Source #1 (Radvaz): The Beraisa (Kidushin 32a) discusses correcting the father while he transgresses, i.e. before Teshuvah. Nevertheless one may not pain the father through words. All the more so he may not hit or curse him! Regarding honor and fear it does not say "Amcha".

Source #2 (Or Some'ach): One can become a Ben Sorer u'Moreh even if his parents are forbidden to each other by Kares or Misas Beis Din and are still 'married' (Sanhedrin 71a). Unless we devise an exceptional case, this shows that they are Resha'im. Ben Sorer u'Moreh is liable for neglect of Kibud Av v'Em, for we find that they can pardon him if they want.

Question (Tur): The Gemara (Bava Metzi'a 62a) says that if the father did not do Teshuvah, his children need not return a stolen cow that they inherited. This shows that one must honor his father only if he did Teshuvah, but not if he is a Rasha!

Answer #1 (Radvaz): That is when he died without Teshuvah and cannot fix his Aveirah. If the father is alive the child must honor him lest he do Teshuvah, and retroactively the child transgressed not honoring him!

Answer #2 (Kesef Mishneh): The Gemara does not discuss a stolen cow. Rather, it was received for Ribis. The Torah commands only the lender about Ribis, not his son. If the father repented and wanted to return the cow but was not able to, we consider it is as if he did not bequeath it to his sons. Otherwise, it is their property, and we hold that a son need not use his money to honor his father. He must show honor when there is no loss of money, even if the father is a Rasha.

Question #1 (Lechem Mishneh): If so, according to the opinion that one must use his money to honor his father, how can we answer? It is difficult to say that he establishes the Beraisa to be when he did not do Teshuvah.

Question #2 (Machaneh Efrayim Malveh 8): According to the Kesef Mishneh, the children do not return for Kibud Av v'Em, rather, because it is as if they did not inherit the property. If so, this should apply even when the father left money, and to any heir, not just children!

Question #3 (Taz YD 240:17): The Gemara asked that the son should not need to return the cow because the father is not "Amcha". According to the Kesef Mishneh, he is exempt because it is the son's property!

Answer #3 (Lechem Mishneh): After the father died one must honor him only if he did Teshuvah. During his life one must honor him even if he is a Rasha.

Answer #4 (Machaneh Efrayim): Beis Din does not force children to fulfill Kibud Av v'Em because the Torah specifies the reward. But if the father left a specific item it forces, for then there is disgrace (Tosfos Kesuvos 86a DH Pri'as). If the father did not do Teshuvah, we are not concerned for disgrace.

Question: The Rambam (5:13) does not allow disgracing a father even if he did not do Teshuvah!

Answer (Mishneh l'Melech Hilchos Malveh 4:4 DH v'Yesh): One may never to an act to disgrace his father. One must act to save the father from disgrace only when he must honor him, i.e. if the father did Teshuvah.

Poskim

Shulchan Aruch (YD 240:8): A Mamzer is obligated to honor and fear his father. even if his father is a Rasha, he must honor and fear his father.

Bach (YD 240 DH Kosav): It is a Mitzvah to disgrace Resha'im. Nevertheless, a son may not be a Shali'ach of Beis Din to hit his father. It seems that this is mid'Rabanan, that one must honor a parent, even a Rasha. However, the Rambam connotes that it is mid'Oraisa. Bereishis Rabah (11:32) says that the Torah mentions the death of Terach before saying that Avraham went to Eretz Yisrael to avoid publicizing the fact that Avraham did not honor him. This seems to show that one must honor an evil parent. However, the Rambam learns that even though one may not hit or curse him, he may passively refrain from honoring him, even mid'Rabanan.

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

Source (Darchei Moshe 6): The Mordechai, R. Tam and Hagahos Maimoniyos (6:7) agree with the Tur.