Worse than Shinzon: "I'm a big evil human clone of Picard. I'm going to destroy Earth. Just because."
That was actually the worst part of Shinzon. They tied his justifications into knots, just so he could threaten Earth, becaus eyou know, it's always gotta be Earth. I've often thought Nemesis would have been a halfway decent movie if Shinzon and the Remans were fighting for their freedom from the Romulans, but using despicable tactics. Instead of a petulant little Napoleon who resents Picard, he could have been like an idolizing younger brother, not understanding why Picard would stand between him and "justice."