Shinzon was insane. Raised in a mine, subjected to constant abuse and taught to hate what he is. If you've had any experience with mentally ill people, you'll find their motivations don't always stand up to scrutiny. IMO Shinzon wanting to wipe out Picard and humanity because they lived such privalaged lives while he suffered endlessly, and because he's a copy of a great man and would never fit in there, was believable. It didn't make rational sense, but it wasn't meant to.