Assuming the plot rumor about Nero killing Kirk's parents (or at least his father) is correct, how do we know that these specific deaths were premeditation on Nero's part? Perhaps George Kirk's death is just the "collateral damage" of some otherwise unconnected scheme of Nero.
That's my take. I'm hoping that Nero is somewhere between reckless and psychotic, and he's just running around fucking shit up because he can. Sort of like Doomsday in the Superman comics (before they overexplained and ruined his origin).
It just makes no sense for him to go all the way back to Kirk's time and try to eliminate him. Even if he is somehow pissed about something Kirk has done, eliminating Kirk would have so many unpredictable effects by Nero's time, that no reasonable man would try it.
There could be an emotional connection to Kirk, like with Khan, but Nero seems too remote from Kirk, time-wise.
So, I hope he's just reckless, like Lobo, or just plain old frakkin' crazy.