No man considers himself evil.

A confession that you stole will not be accepted to convict you as a thief. You will be required to give him the money by way of a confession by a party. A confession by a party is accepted regarding the money, but the implications regarding the person (wickedness) are not accepted because he is a witness and is disqualified from testifying. There are several explanations for the difference between this and a confession in writing. First, some argue that a confession in writing is not a trust, but a type of gift. In any case, even if it is a trust, it is clear that there are no witnesses involved. In financial matters, a confession in writing is useful because evidence other than witnesses is useful, but with regard to a person’s conviction or disqualification from testifying, evidence, no matter how good, is useful, but only witnesses. See Rambam, Hala Sanhedrin, beginning of P.C. and beginning of P.C.