I must say that I'm a little bothered by your seeming disregard for even the implied loss of life in a scenario like this. Why do you believe that it's acceptable to commit genocide simply because it accomplishes a military objective? What's your justification for such an action?
Wars are not conducted with preset protocols or etiquette that opposing sides are obligated to follow. It's not a sport where there are rules and a referee who will call out violators of each side.
I never said genocide was acceptable. However if the Romulans wanted a victory, and genocide of Earth would achieve such a goal. What would be the harm in letting Shinzon carry out his plan? Who is going to hold the Romulans accountable when they are ruling the remaining Federation worlds? Donatra and Suran couldn't live with themselves if such a thing happened so they left to fight against Shinzon. However out of the entire Romulan fleet only two ships went to stop Shinzon. Two. I'm not judging the race on a whole but sending so few ships? The rest of military seemed indifferent of the result whether Shinzon succeeded or failed.