"Guilt," in its present incarnation, is associated with accountability and responsibility: you are guilty because you could have and should have done otherwise. Accountability and responsibility, which are connected with the concept of free will, are in no way connected with "guilt" as it was originally conceived. "Guilt," according to Nietzsche, originally meant simply that a debt needed to be paid. As Nietzsche remarks in section 13 of the first essay, "free will" is a recent invention that accompanies slave morality.
Punishment, according to slave morality, is then meted out because, and only because the offender could have acted otherwise. If someone is for whatever reason deemed not to have acted freely (insanity, duress, accident, etc.) they are not punished.
Nietzsche's conception of the ancient world is far crueler, but, he suggests, far more "cheerful." People were punished simply because it was fun to punish people. If you fail to keep your promise to me, at least I get the pleasure of beating you up. Here we see the original association of "guilt" with "debt." Guilt was seen as a debt to be paid: if you make a promise, you are in debt to me. If you fail to keep your promise, you must pay off the debt in some other way. If that "other way" is my gouging your eye out, there are no hard feelings afterward, and there is no sense of a corrective measure being taken. There is simply an agreement that now our debts are settled and we can go our different ways.
It is quite easy to understand why Nietzsche would characterize an age of torture, mutilation, and delight at the suffering of others as "cruel," but it might be harder to understand why he might characterize it as "cheerful." The key might be found in the suggestion that there would be no hard feelings between "creditor" and "debtor." Our modern concepts of morality plunge us into a bog of "bad conscience." We are constantly being watched and judged, we are always watching ourselves to be sure we are behaving appropriately. None of this was present in Nietzsche's conception of ancient societies. Our lack of cheer today stems from the fact that our wrongdoings and our guilt stay with us and plague us. In ancient times, one would submit to punishment and that would be the end of it. For most of the time, the ancients did not trouble themselves much about what they ought to be doing or about whether they had done wrong. They lived free of moral torments and were thus more cheerful.
Nietzsche presents frustratingly little evidence for his claims about how things were in times past. In a sense, he is much like Freud: his imagination and his genius far outstrip his carefulness as a scholar or his interest in empirical evidence. In other of his writings, Nietzsche often comes down quite harshly against the British empiricists and their methods, so it is understandable that he would not want to proceed in an empirical spirit. Still, we might find ourselves feeling a little more inclined toward the careful defense of empirical claims when we consider how little Nietzsche's arguments would stand up if his unsupported historical claims proved false.