Red Matter was not originally designed to be used as a weapon, but a way to save a dying star's planets. If given at the dosage and time, it would have caused a blackhole, but not necessarily sucked Romulus & Remus in as well. Yes, the Romulans still would have lost their homeworld due to their sun turning into a blackhole. But they would have had the chance to escape. At least that is how it should have worked theoretically.