Either way, Nero didn't have what he needed to exact his revenge on Vulcan. That was his main thing. He hated Vulcan and Spock in particular. The fact that eradicating them would save Romulus in a timeline he couldn't return to wasn't exactly part of his agenda at first. He probably decided upon destroying the Federation after waiting around 25 years(and somehow being captured by Klingons) for Spock to return. Once he captured the Jellyfish and the Red Matter(such a descriptive term, and if you think about it, very scientific, eg. dark matter, big bang), Nero had the tool he needed to exact his revenge. See, Nero probably could've laid waste to the surface of Vulcan, but I think he finds it poetic justice to wipe it out of existence.