But Shinzon conquered Romulus. He was the freakin' Praetor! They're his subjects now, after a life of being opressed by them. That's far better than killing them.
One of the weakest points of the film. He casually assassinates the entire government of a planet with probably billions of people, and nobody cares, and nobody cares about those billions who don't care. Stuff like that is what diminishes the scope of a film's story. And that's also where the lack of a Spock storyline became badly apparent. He wouldn't just sit back doing nothing if something like this happened.