you don't think the idea of Shinzon accomplishing the defeat of the Federation, something the Romulans couldn't on their own appealed to him?
His thought process could have been "I, a discarded clone sent to die accomplished something that actual Romulans never could."
I can buy the idea that he'd want to take on the Federation at some point, if only to increase his relative power. What I don't
buy is that he'd direct a genocidal impulse towards the Federation. That's what's arbitrary -- not that he'd want to defeat the Federation, but that he would want to exterminate Earth before Romulus. This is a guy who feels nothing but contempt for the brutal oppressors who enslaved him and everyone he loves; it's completely inconsistent with his characterization and his established motives that he'd decide to go after Earth instead of Romulus.