ok, you may not like the direction they took Shinzon's character in. But in what way did the choices he made in the third act "defy the set of motivations" he was established to possess?
How many more times do I have to repeat myself?
Shinzon's motivations are all about Reman nationalism: He wants to liberate the Remans and is against the Romulans. His primary motivation relates to the Reman/Romulan relationship, to his desire to shatter the Romulan system of control.
for extermination rather than Romulus defies everything we learned about his political views and personal motivations. It's done for the sake of having "Earth In Danger" inserted into the script rather than allowing the character to behave as he would logically given his established motivations.
um, no that's not "defying his motivations." Defying previously established motivations would be if he were shown previously that his goal was PEACE with Earth, and then he turns around and tries to attack it. No move he makes against Earth or against the Ent-E contradicts anything previously established about his character.
We already saw at the beginning that he was a ruthless mass murderer with a grudge against the Romulans. But he already MOVED against them, he assassinated their senators and took over their government. He won.
So... now he's moving on a new target and in doing so he's getting revenge on Picard in his own mind. Again, just because you don't like the direction they took his character, just because you wanted a different movie where Shinzon goes after Romulus, doesn't mean that his motivations are arbitrary.