However the difficulty is a supernova can’t commit genocide.
Very good point, and very good followup argument.
Who can? Nero could indeed be thinking that there was an active conspiracy to ensure the death of Romulus in a natural disaster, or even that the disaster was engineered by evil forces. And what had Nero done that could plausibly have been directed against such evil forces, before yelling to Pike about it?
Well, he had killed Vulcan, which is probably what he's referring to, even though his language and logic is confused (and forces us debate it here, which the writers probably wouldn't have wanted). But he had also engaged the Klingons in massive combat, seemingly for no reason. And if we can trust our Starfleet heroes and cronies with their facts, he
first acquired red matter from Spock, and
then, an hour later, engaged the Klingons and defeated their fleet.
We lack explanation as to why Nero would want to attack the Klingons. Perhaps this seemingly unjustified attack was related to him "preventing genocide", too? Could be he thought the Klingons were involved in the future calamity. Could be he thought the Klingons were attempting genocide against Romulus in the 2250s, and he included defeating them in his collection of good deeds to do in that timeframe. Or could be that defeating the Klingons was a necessary step in thwarting the evil plans of the Vulcans - possibly because the supernova star was in or near Klingon territory and could only be neutralized by neutralizing the Klingons first.
Remember, it's not only Romulus that is threatened by Hobus.
Spock might be a bit poetic about it, actually thinking that loss of Romulus would have galactic repercussions. We don't know if he considers his mission a partial success (because he stopped the supernova even if after loss of Romulus) or a complete failure (perhaps he had to use his primed red matter anyway, lest it blow on his face, even though all was already lost that was going to be lost).
The latter would be the more real-worldish option, because real supernovae don't threaten galaxies, and it's a bit difficult to see how fictional ones could, either. The former might be writer intent, though. But we're faced with intriguing evidence there, such as why Spock only had seconds to spare after loss of Romulus. Surely there would be minutes or hours until the wave hit some other star system, the way it was depicted? Why were the seconds following the loss of Romulus critical? Could it be Spock was actually doing relatively small-scale stuff, and trying to save Remus or other outlying worlds in the Romulan system, from the explosion of a very local supernova?
In a way, the smaller scale would make things even more personal for Nero...
Timo Saloniemi