Well, yeah, but he didn't know any better. Nero was a good, honest man who lost everything. Shinzon grew up in a mine and won his freedom by focusing his work on overthrowing the Romulans. Then, suddenly, he decides to destroy Earth...for what reason?
Shinzon really didn't bother me until the back half of the film when he started dying and went crazy. Normal people don't continue walking after they've been impaled by random spikes.