Gemara

(Mishnah): If Reuven says 'Leah's husband died', 'I killed him' or 'we killed him', he may not marry her;

R. Yehudah says, if he says 'I killed him', she may not marry. If he says 'We killed him', she may marry.

Inference: Chachamim forbid Reuven to marry her, but others may.

Question: But Rav Yosef taught that if Shimon says 'Ploni forcibly had Mishkav Zachar with me', Shimon may join a second witness to condemn Ploni to death;

If he says 'I willingly had Bi'ah with Ploni', Shimon is a Rasha (and Pasul for testimony) - " Al Tashes Yadcha Im Rasha Lihyos Ed Chamas".

Answer #1: Perhaps Edus Ishah (testimony to permit a widow to remarry) is different, for Chachamim were lenient.

Question: Rav Menasheh taught that a thief mid'Rabanan is Kosher for Edus Ishah, but a thief mid'Oraisa is not. Must we say that Rav Menasheh holds like R. Yehudah (unlike Chachamim)?!

Answer: Rav Menasheh can hold like Chachamim. She may marry because of Rava's law.

(Rava): A person cannot establish himself to be a Rasha (and disqualify himself from testifying) because he is Karov to himself (relatives cannot testify).

Suggestion: Rav Yosef holds like R. Yehudah.

Rejection: Rav Yosef can hold like Chachamim. He says that we are more lenient about Edus Ishah than about normal testimony. Rav Menasheh holds like R. Yehudah!

Kesuvos 18b (Mishnah): If witnesses on a document say 'these are our signatures, but we were forced to sign', they are believed.

(Rami bar Chama): They are believed only when they say that they were mortally threatened, but not if they say that they signed to save their money.

This is because a person cannot establish himself to be a Rasha.

Sanhedrin 9b (Rav Yosef): If Shimon says 'Levi forcibly raped me', Shimon can join with a second witness to kill Levi. If he says 'I willingly had Bi'ah with him', he is a Rasha (and cannot testify) - "Al Tesht Rasha Ed".

(Rava): A person is Karov to himself, so he cannot establish himself to be a Rasha. (Therefore, we split his testimony and ignore his admission that he consented, and he joins with a second witness to kill Levi.)

Sanhedrin 25a: Ploni testified 'I saw Bar Binisus borrow on Ribis', Almoni testified 'I borrowed from Bar Binisus on Ribis'; Rava disqualified Bar Binisus.

Question: But Rava said that one who borrows on Ribis is disqualified, therefore, Almoni is a Rasha - "Al Tesht Rasha Ed"!

Answer: Rava ruled according to another of his teachings, that a person is Karov to himself, therefore he cannot establish himself to be a Rasha.

Rishonim

Rif and Rosh (Yevamos 6b and 2:10): If Reuven says 'I killed Leah's husband', he may not marry her, but others may. The Halachah follows Rava, who says that a person is Karov to himself, so he cannot establish himself to be a Rasha.

Rif and Rosh (Kesuvos 6a and 2:8): If witnesses signed on a document say 'these are our signatures, but we signed due to a monetary threat' they are not believed, for a person cannot establish himself to be a Rasha.

Question: When witnesses say that they signed (a false document) due to a monetary threat, we should split the testimony and believe that they were threatened, but not that the threat was monetary (rather, it was mortal and they were permitted to sign)!

Answer #1 (Ran Kesuvos 6a DH Gemara): We split testimony only if we need not change what they said (we merely ignore part). If we would say that they were threatened mortally, this is unlike what they said!

Answer #2 (Nimukei Yosef): We split testimony only in cases such as 'I killed (or willingly had Bi'ah with) Ploni'. It is two separable testimonies; it is possible for one without the other (I killed someone, or someone killed Ploni). But they cannot say that they were forced to sign without explaining if the coercion was mortal or monetary. Therefore, we do not split this testimony.

Answer #3 (Mordechai Yevamos 2:16): We split testimony by ignoring part of what was said. We do not fabricate things that were not said. Also, any Ones other than Nefashos invalidates their testimony, so we do not adopt the minority explanation.

Answer #4 (Tosfos Sanhedrin 9b DH v'Ein): This is like the opinion (Bava Basra 134b) that does not say Palginan Diburei in one Guf. Alternatively, mid'Oraisa a signed document is like testimony that was investigated and accepted by Beis Din. Only mid'Rabanan one must validate a document. Therefore, we do not say Palginan Diburei to disqualify a document.

Answer #5 (Kesef Mishneh Hilchos Edus 12:2): When a witness says that he was forced to sign, his primary testimony is that he did not see the loan. Therefore, he must say that he was mortally threatened to avoid establishing himself to be a Rasha. Therefore, we cannot divide his testimony. When a witness says that he had Bi'ah with Ploni, his primary testimony is that Ploni should be killed. Therefore, we divide his testimony so he will not establish himself to be a Rasha.

Question: We hold that if testimony is partially Batel, it is totally Batel (Makos 5b)!

Answer (Nimukei Yosef): None of the testimony is Batel. His admission that he killed or had Mishkav Zachar is not testimony.

Rif and Rosh (Sanhedrin 4b and 3:8): Rava disqualified a man from testimony based on two witnesses, one of whom said that he borrowed from Bar Binisus on Ribis. Even though Rava disqualifies one who borrows on Ribis from testifying, a person cannot establish himself to be a Rasha.

Rambam (Hilchos Gerushin 13:10): If Reuven says 'I killed Leah's husband', she may remarry. One cannot establish himself to be a Rasha, and Reuven already testified that her husband died.

Rambam (Hilchos Edus 3:7): If witnesses are signed on a document (even if we cannot validate it without them) and they say that they were invalid witnesses or were bribed to sign, they are not believed. One cannot establish himself to be a Rasha. Only others can do so.

Rambam (12:2): One who says that he borrowed from Ploni on Ribis can join another which to disqualify Ploni.

Poskim

Shulchan Aruch (EH 17:7): If one said 'I killed Leah's husband', she may remarry, for a person cannot establish himself to be a Rasha, and Palginan Diburei.

Chelkas Mechokek (16): Some text of the Tur says 'if one of these said, e.g. even a slave (mentioned in Sa'if 5). Our texts say 'If one said'. According to this, perhaps we do not say Palginan Diburei regarding a slave.

Beis Shmuel (20): Palginan Diburei and we say that he means that he was with those who killed him.

Teshuvas Rashba (1237): If a witness says 'Ploni's wife was Mezanah with me', we can ignore 'with me' and keep the remaining testimony that she was Mezanah. If he says 'I killed Ploni willingly', we can ignore 'willingly' and say that he killed him b'Ones or b'Shogeg. But if he says 'Bo'alti Ploni's wife' or 'Bo'alti Ploni', if we omit 'Bo'alti' no testimony remains. Kishuy is b'Ratzon, so there is no normal case of Ones (or Shogeg). Therefore, such testimony is void. This is why the Gemara discussed specifically 'Ploni was Bo'el me', which is valid (we ignore 'me').

Be'er Heitev (24): If Reuven said 'My brother (or another relative about whom Reuven cannot testify) killed Leah's husband', we are lenient to say Palginan Diburei and let her remarry.

Shulchan Aruch (CM 34:26): If Reuven testified 'Michah lent to me on Ribis', and another witness testifies with him, they disqualify Michah. Even though Reuven incriminates himself, Palginan Diburei, and we believe him only about Michah, not about himself.