Rufus decided the only way to approach Beth would be in person. That way he could judge through body language if she would turn him in to the police if she realised he was alive after all. It would certainly be a shock for her.
She had gone to his funeral, which he took as a sign that she'd still had feelings for him. After his sex attack, when she fell pregnant, Rufus had shown himself to be concerned and offered support. It was possible, therefore, that she might not turn him in, immediately.
Rufus needed to approach her incognito.