At least the destruction of Alderaan had a tactical purpose: it was meant to intimidate the Empire’s enemies into submission to ensure a peaceful (evil & totalitarian) reign.
Nero, on the other hand, didn’t have any apparent goal beyond being a huge dick. He obviously didn’t care about helping the Romulan Empire or he would have just headed back to Romulus to share his fancy futuristic technology with his people, maybe just use some of that black hole-juice to stop the supernova which was destined to destroy his planet.
