Sorry, let me explain this way. Say I managed to talk my wife into playing mafia and then say I was chosen to play the ancient vampire and had the choice to choose my slaves, then I would consider choosing my wife as one of them. Why? It would give her the chance to be on the mafia team and it would allow me to help her. As far as I know Diana has never been mafia, so if ATPG were the ancient vamp, this would be a great time to both give her the opportunity and help her if she is in need of it. So his never accusing her may not mean she's not guilty, it may simply mean he doesn't wish to accuse his mafia teammate.