I think there's more subtext here than we care to admit. People tend to dismiss the movie as "bad" without really understanding it.
Shinzon goes after Earth because it was promised as a prize to the Romulan fleet that helped his rise to power. As Riker states, "The Preator's power has always been the Romulan Fleet." That is why there is constant prodding from the Romulans for him to attack the Federation. That said, there is a deleted scene they needed that would've made that clearer.
There is a purpose to Shinzon's "loose characterization." The man is conflicted. He doesn't hate Picard. He wants to be better than Picard, prove he is worthy of being alive. Shinzon has had no contact with humans. Picard is almost a father figure through most of this movie. Shinzon is contemplating whether or not he should destroy Earth and Picard, or be peaceful and embrace his human heritage. He's acting more like a badass than he really is.
The "Reman brothers" that "showed him the only kindness he has ever known," help him rape Deanna Troi. It's more about showing what he will be choosing if he embraces the Remans. "We are a race bred for war...and conquest." He's simply misguided and conflicted throughout the movie. He's not a usual villain.
Up until his final moments, Shinzon is competing with Picard to show him who is going to be better in history books, and who has the power and the resolve, even after Picard has gone through him with a spear, and defeated him. He destroys himself because his circumstances never allowed him to feel love or empathy or compassion. The world taught this
Picard that he must become a warrior to survive, and to hate the Romulans, use them as pawns.
The effect of Shinzon's presence on Picard is that he sees what he could become, what he thought, because it's the 24th Century, he couldn't possibly do. Remember--war, poverty, disease, hatred, suffering--it's all gone. The Borg and the Dominion have brought it back, and Shinzon rises to be a great military leader (important to the Romulan fleet) because of his engagements in the Dominion War. "The raw material is the same." Shinzon makes Picard hesistate because he truly believes that Shinzon is a mirror for Picard. Given the right circumstances, Jean-Luc Picard could become the next Hitler. His greatness lies in his circumstances. And, indeed, Picard is facing the fact that he could commit genocide with the Borg. "It's not a person, Damnit, it's a Borg!," "I will make them pay for what they've done!," etc. That is the dark side of Jean-Luc Picard. It's consistent with his character.
The moral of the story is two-fold: "Violence breeds violence, Oppression breeds retaliation
." -- Robert Kennedy. The second is that we have the ability to do wonderful and terrible things to each other. We are capable of killing en masse, or being peaceful and benevolent. Shinzon is more successful than Picard
. He is the leader of a planet. But he chooses genocide over peace.
In the end, Picard is left asking Data about his humanity, an interesting turn-of-fate. Data has come full-circle. He used to challenge Picard to talk about humanity and his uniqueness. Now, Picard turns to him and says "I need to know if I could really do this." And Data responds. "I aspire, sir, to be more than I am. B-4 does not. Nor does Shinzon." But ultimately, we have to make the conscious choice to overcome circumstances. And that requires nurturing thought, foresight, and morality early in childhood, as well as the genetic components that allow us to have morality and higher thought.
The movie is better than you think. I rank it between Insurrection and Generations as my 2nd favorite TNG film. It's too much action for me to make it the best. It took me awhile, and more experience, to fully understand the film. But that is my take on it. Others, I'm sure, have different opinions.