I think Rudy knows he already admitted to being there, and to being sexual with MK. This is his reason, of course...Rudy testifying as a witness means he can get sued for slander and get years added to his sentence if he lied. Besides it would have been in his best interest to claim both AK and RS to be innocent. In that case he could try and jump on the 'all forensic evidence is invalid' train. However he didn't lie and his testimony is a very serious piece of evidence.