Rashi says that if the witnesses say that they were forced Machmas Mamon then they are not believed because they are not believed to make themselves into Rehsha'im and rashi continues and says d'Adam Karov Etzel Atzmo. My questions is why does rashi have to come on to this last point of she'Adam Karov Etzel Atzmo but without this they are not believed because they are not believed to make themselves into Resha'im and my question is also the other way round that why can't rashi just say that the reason why they are not believed is because he is considered to be a Karov on himself and it won't be a problem on the last part of the mishna which says they are believed because there they have a Peh she'Asar? Basically my question is why does rashi have to come on to 2 reasonings
You are discussing two different Halachic reasonings. The concept of Adam Karov Etzel Atzmo is used in the laws of testimony to explain why a person's testimony cannot be relied on regarding himself. This principle technically applies to every testimony --even positive -- of a person regarding himself, but an extension of this is that a person cannot incriminate himself. What Rashi is saying based on the Gemara (in a number of places which brings both principles together) is that because of the first principle, the second one follows logically. This is how Rashi explains this in Sanhedrin 9b.
The second principle enables us in other places in Shas to ignore the incriminating aspect of the witness' testimony regarding himself while accepting his testimony in principle and using it to incriminate others. In our case, this is not possible for a number of reasons, as cited by Tosfos (DH Ein Adam).
Yoel Domb