In Jewish law can a defendant or an accused person be a witness against himself? If there is a prohibition on use of such confession, would this prohibition only be for non-capital (death penalty) cases? If there is such a prohibition on the use of such a confession what is the reason behind it?

In criminal matters a person's testimony against himself or herself is invalid. The Talmudic reason is that since the testimony of a relative is invalid, any person is related to himself. The broader reason is that in Jewish law testimony may only come from objective sources. A good friend or enemy is also an invalid witness, as is a criminal.