M'rk, son of Mogh wrote:
It just doesn't make sense that Shinzon would direct his genocidal urges at anyone other than the people who had actually enslaved him.
Since when do people who do horrific evil deeds make SENSE?
Most people, even genocidal war criminals, have motivations that are discernable, even if their actions are irrational.
Shinzon's motivations are dramatically arbitrary; he does not target Earth because the Federation has ever done anything to him. He targets Earth because the plot demands it, irrelevant of whether or not it's consistent with the rest of his depicted motivations.