TJ Sinclair wrote:
And because he hated the Romulans, he wanted to destroy all life on Earth...
, not the most logical of plots.
No, that was because he hated Picard, hated being a mere echo of a great man, and thus wanted to destroy that man's accomplishments and everything he held dear. Shinzon explained his motivations perfectly clearly in dialogue: "I'll show you my true nature. Our nature. And as Earth dies, remember that I will always, forever, be Shinzon of Remus! And my voice shall echo through time long after yours has faded to a dim memory."
At its core, the emotional arc of NEM was about Shinzon and Picard, not Shinzon and the Romulans. The thematic arc was about nature vs. nurture, about whether the two of them were really the same being by genetic destiny or were shaped by their choices -- with Shinzon blaming his own bad behavior on his genetics or his circumstances or his persecution and doing everything he could to dodge responsibility, while Picard was the better man because he faced his own faults and took personal responsibility for bettering himself. But because Shinzon believed in nature over nurture, believed that he and Picard were effectively the same person, and hated Picard for getting all the glory and triumph and happiness while Shinzon was condemned to slavery and suffering and aloneness. The Romulans were just one of the circumstances he blamed his choices on, the reason for the lousy lot in life that he was using as the excuse for his behavior. But Picard was his main target, because he couldn't think of himself as anything more than a copy of Picard until he destroyed the man and his accomplishments, until he effaced Picard's name from history and carved his own across it.