^Think about the two villains who have the most in common with Shinzon. Khan Noonien Singh and Nero.
Khan wanted the Genesis torpedo for unspecified reasons. We the audience were told what Genesis was capable of and we were shown Khan is definitely a villain. Letting Khan have Genesis is like giving an arsonist matches. We have to keep them away from him at all costs. While you could argue since Khan's intentions for the Genesis were never stated the movie is lacking. I think the quest and battle for it is what save TWOK from that trap.
Nero lost his planet and went on a tirade against the Federation in an alternate timeline set 125 years in the past of the one he came from. Vulcan and Earth had nothing to do with Romulus blowing up in the Prime future but he attacks them anyway for his own narrow minded reasons. Logically Nero should've went to Romulus in this alternate timeline to warn his people. Instead he waited for 25 years to get revenge on Spock. This is dialogue straight from the movie, and until JJ confirms on screen Nero and the Rommies were at Rura Penthe, the canon is Nero waiting out in space for Spock to show up. Nero attacking the Federation planets in this past timeline makes no sense because it does nothing to stop the supernova that will hit Romulus in 125 years time.
Shinzon was the legitimate Praetor of the Romulan Star Empire. You could argue he was merely a figure head and his Romulan collaborators intended to betray him but that's only speculation. Shinzon states his goal is conquest. As a military leader with dozen of battle victories over the Dominion. The logical course of action for any Romulan victory in the Alpha Quadrant is the destruction of Earth. It's the head of Starfleet and focal point of the Federation. Picard, Riker and Shinzon all state and thereby confirm this is Shinzon's motive for attacking Earth.
Comparing Shinzon to Nero in this regard. Shinzon has a degree of class and sense to his actions. Destroying Earth is a tactical move in his future campaign against the Federation and the Klingon Empire. Nero is a blunt object attacking those he thinks deserve to die. The Narada has transwarp drive. Confirmed by the special features of the DVD. Why Nero didn't just make a quick hop home to Romulus and share his future tech and warn the council of the impending danger is beyond me.
I guess when you're just a "simple miner" comprehending the balance of power on a galactic scale is too much for you. Shinzon was trained by the League of Shadows, took down Batman, and Gotham. He's the man with the plan. Haha