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.