Shinzon is pretty obviously emotionally and possibly mentally disturbed, and his attack on Earth was a way-in his own sick mind-of validating his rule as Praetor. He also was feeling resentment at Picard being better than him in some way, and so again, he attacked Earth.
This is from my response to the "rewrite Nemesis" thread, with extras:
One of the main problems of this movie was the total lack of explanations for things.
1. Who the hell were the Remans, and why the hell have we never heard of them before? Were they offshoots of the Romulans, or an alien race that was already living on Remus when the Romulans colonized Romulus? The film gives no explanation whatsoever. And if they were the original Romulan settlers, why do they look so different from Romulans? There's no way they should have evolved to look that different from Romulans in only 2,000 years, no matter how harsh their planet was. Obviously they look the way they do for "shock effect," basically "they MUST be evil if they look like that!" (which is the direct opposite of what Star Trek is all about) but they ended up as cardboard villains. It's hard to feel sympathy for them being enslaved when they came across like that.
2. What was the point of the Romulans cloning Picard twenty years ago? From what I remember, it was supposed to be an accelerated-aged Picard, so that he'd be the same age as the real Picard in just twenty years. But why would they have thought Picard would be important twenty years down the road? And how did they get Picard's genetic material twenty years before?
3. What's Shinzon's motivation? He talks about freeing the Remans and taking revenge on the Romulans, but then all of a sudden he wants to attack Earth? Why? What did Earth ever do to him? Unless Donatra and her allies wanted to attack Earth, but why would Shinzon really care what they wanted, since he was only using them to get what he wanted? (I know he also needed Picard's blood, but if he had half a brain in his head he would have known that Picard would have given him a transfusion had he simply asked for it, instead of his convoluted plot to steal it from him.) I'm not buying your "mentally disturbed" excuse for Shinzon. Sure he's young and impulsive (just like the real Picard when he was younger), but I don't see any mental issues with him. It just boils down to the fact that he's just a stupid kid.
4. And on the subject of convoluted plots: Shinzon finds B4 (again without an explanation as to how), and decides to use him both as a lure for Picard and a way to access the Enterprise's info. So the point here is to get B4 into Picard's possession quickly. So how does Shinzon do this? By breaking B4 up into different pieces and planting these pieces all across a planet with incredibly hostile natives, making it extremely hard for Picard to retrieve B4. What kind of a stupid plan is that? Why didn't he just place B4, intact, floating in space in the Enterprise's flight path for Picard to easily pick him up? (Because then we wouldn't have had that stupid dune buggy scene).
5. And on the subject of the Argo Jeep...there must have been several more logical ways to retrieve the B4 parts without endangering Picard's, Data's, and Worf's lives in the process. Hell, if anything, send down three redshirts to drive around that hostile planet!
6. Where did Shinzon get the raw materials to build the Scimitar, and why didn't the Romulans know about this really large warship being built right under their noses? And did the Remans build it in their spare time or something, as if a slave has any spare time?
Would you like some more examples?
