It might be different if the two of them were arguing, the neighbor lost it and made a regrettable error in judgment in the heat of that argument. But according to the story, Rand Paul was mowing the lawn when the neighbor jumped him and started wailing on him. Looks to me like there was no imminent danger to the neighbor, no provocation and the attack was premeditated. Under those circumstances, I don't see any mitigating factors. Unless the two of them can settle their differences beforehand or the prosecutor decides it's a waste of time and offers a plea deal, this guy should do some serious jail time. If I were Rand Paul, however, I would ensure there was no deal.
And by the way, I'm not a Rand Paul supporter.