If somone admits that they are a invalid witness do we believe them? Does it matter whether they're saying they're invalid because of a Aveira or a bad midda?
אֵין אָדָם נִפְסָל בַּעֲבֵרָה עַל פִּי עַצְמוֹ. כֵּיצַד. הֲרֵי שֶׁבָּא לְבֵית דִּין וְאָמַר שֶׁגָּנַב אוֹ גָּזַל אוֹ הִלְוָה בְּרִבִּית. אַף עַל פִּי שֶׁמְּשַׁלֵּם עַל פִּי עַצְמוֹ אֵינוֹ נִפְסָל. וְכֵן אִם אָמַר שֶׁאָכַל נְבֵלָה אוֹ בָּעַל אֲסוּרָה אֵינוֹ נִפְסָל עַד שֶׁיִּהְיוּ שָׁם שְׁנֵי עֵדִים שֶׁאֵין אָדָם מֵשִׂים אֶת עַצְמוֹ רָשָׁע.
A person does not become invalid [to testify] through committing a sin on his own testimony. How so? Someone comes to court and admits to burglary or robbery or to lending with interest, even though he has to repay based on his admission, he does not become invalid [to testify]. Similarly, if he admits to eating neveilah or having sexual relations with one forbidden to him, he does not become invalid unless there are two witnesses [to his crime], because a person cannot deem himself to be wicked.