Shinzon was mentally ill, after a life of terrible abuse. His want to destroy Earth and Picard, to drag Picard down and prove himself equal to them fits with paranoid behaviours.
I wonder if those who refuse to accept Shinzon's motives have any experience with the mentally ill?
See also: Hitler. The legend goes that a Jew beat him out of a place in art school, thus: attempted genocide. Yet nobody calls him unrealistic.