That was Shinzon's problem. He did not only want to execute revenge on the Romulans, but on everyone because he felt abandoned.
But that's not really true. He may have felt abandoned, but that was certainly not his motivation for attacking Earth. The only reason why he eventually decided to attack Earth was because the renegade Romulans kept pressuring him to. Romulans, I might add, that he really had no obligation to. Yes, they helped him kill the Senate and take over the planet, but once that was accomplished, Shinzon could have just offed them too. But instead he let them boss him around, even though he was in charge! Just one of many things fundamentally wrong with this film.