In Nemesis Shinzon is pissed at the federation to an extreme. But the federation never did anything to him, the only ones responsible for anything bad happening to him are the Romulans. The federation never knew he even existed.
Well this is ultimately why the movie fails overall despite some decent moments. It never gives us any reason why the hell Shinzon is so pissed at the Federation, or Picard for that matter.
I guess maybe he's jealous that he and Picard are the same person yet Picard had a successful and comfortable life and he knew only pain and misery.
And perhaps he just wanted to destroy earth to show the Romulans and the rest of the galaxy what a bad ass he is.
Neither reason is all that compelling though. Especially when it has so many similarities to TWOK and Khan had the most personal reasons of all for hating and wanting revenge on Kirk.
Seriously what was Shinzon's deal. I guess he was just an evil asshole. Sure they exist in life, but they don't make for very compelling antagonists in films if that's their only motivation for being bad.
Someone among them must have said,
"So far this movie sucks!"
"What could we do to make it better (suck less)?"
"I have an idea! Let's kill Data!"
"good idea! That will definitely make the movie not suck"
"Yeah!"