This evening I found myself wondering just what ancient sources record the story of the cruelty of Vedius Pollio.
For those unfamiliar with the story, Pollio used to keep man-eating lampreys in a tank. When a slave displeased him, he would order the slave thrown to the lampreys. One day the emperor Augustus was dining with him, when a slave happened to break a crystal cup. Pollio ordered him thrown to the lampreys; but the boy escaped and threw himself at the knees of the emperor, begging to be executed in some other manner than being eaten alive. The emperor sought to calm Pollio, who was implacable. Then Augustus ordered that all of Pollio’s valuable cups should be brought; and when they were, he ordered them smashed. The slave seems to have been allowed to live.
Here are the sources that I can find.
Seneca the Younger, De ira (On Anger), book 3, chapter 40:
To reprove a man when he is angry is to add to his anger by being angry oneself. You should approach him in different ways and in a compliant fashion, unless perchance you be so great a personage that you can quash his anger, as the Emperor Augustus did when he was dining with Vedius Pollio.
One of the slaves had broken a crystal goblet of his: Vedius ordered him to be led away to die, and that too in no common fashion: he ordered him to be thrown to feed the muraenae, some of which fish, of great size, he kept in a tank. Who would not think that he did this out of luxury? but it was out of cruelty. The boy slipped through the hands of those who tried to seize him, and flung himself at Caesar’s feet in order to beg for nothing more than that he might die in some different way, and not be eaten.
Caesar was shocked at this novel form of cruelty, and ordered him to be let go, and, in his place, all the crystal ware which he saw before him to be broken, and the tank to be filled up. This was the proper way for Caesar to reprove his friend: he made a good use of his power. What are you, that when at dinner you order men to be put to death, and mangled by an unheard-of form of torture? Are a man’s bowels to be torn asunder because your cup is broken? You must think a great deal of yourself, if even when the emperor is present you order men to be executed.
Seneca the Younger, De Clementia (On Clemency) book 1, chapter 18:
Slaves are allowed to run and take sanctuary at the statue of a god; though the laws allow a slave to be ill-treated to any extent, there are nevertheless some things which the common laws of life forbid us to do to a human being.
Who does not hate Vedius Pollio more even than his own slaves did, because he used to fatten his lampreys with human blood, and ordered those who had offended him in any way to be cast into his fish-pond, or rather snake-pond?
That man deserved to die a thousand deaths, both for throwing his slaves to be devoured by the lampreys which he himself meant to eat, and for keeping lampreys that he might feed them in such a fashion.
Cruel masters are pointed at with disgust in all parts of the city, and are hated and loathed; the wrong-doings of kings are enacted on a wider theatre: their shame and unpopularity endures for ages: yet how far better it would have been never to have been born than to be numbered among those who have been born to do their country harm!
 Vedius Pollio had a villa on the mountain now called Punta di Posilippo, which projects into the sea between Naples and Puteoli, which he left to Augustus, and which was afterwards possessed by the Emperor Trajan. He was a freedman by birth, and remarkable for nothing except his riches and his cruelty. Cf. Dion Cassius, LIV. 23; Pliny, H. N. IX. 23; and Seneca, “On Anger,” III. 40. 2.
Cassius Dio, book 54, chapter 23 (via Lacus Curtius):
1. This same year Vedius Pollio died, a man who in general had done nothing deserving of remembrance, as he was sprung from freedmen, belonged to the knights, and had performed no brilliant deeds; but he had become very famous for his wealth and for his cruelty, so that he has even gained a place in history.
2. Most of the things he did it would be wearisome to relate, but I may mention that he kept in reservoirs huge lampreys that had been trained to eat men, and he was accustomed to throw to them such of his slaves as he desired to put to death.
Once, when he was entertaining Augustus, his cup-bearer broke a crystal goblet, and without regard for his guest, Pollio ordered the fellow to be thrown to the lampreys.
3. Hereupon the slave fell on his knees before Augustus and supplicated him, and Augustus at first tried to persuade Pollio not to commit so monstrous a deed. Then, when Pollio paid no heed to him, the emperor said, “Bring all the rest of the drinking vessels which are of like sort or any others of value that you possess, in order that I may use them,” 4. and when they were brought, he ordered them to be broken.
When Pollio saw this, he was vexed, of course; but since he was no longer angry over the one goblet, considering the great number of the others that were ruined, and, on the other hand, could not punish his servant for what Augustus also had done, he held his peace, though much against his will.
5. This is the sort of person Pollio was, who died at this time. Among his many bequests to many persons he left to Augustus a good share of his estate together with Pausilypon, the place between Neapolis and Puteoli, with instructions that some public work of great beauty should be erected there.
6. Augustus razed Pollio’s house to the ground, on the pretext of preparing for the erection of the other structure, but really with the purpose that Pollio should have no monument in the city; and he built a colonnade, inscribing on it the name, not of Pollio, but of Livia.
Pliny the Elder, Natural History, book 9, chapter 39 (via Perseus):
Vedius Pollio, a Roman of equestrian rank, and one of the friends of the late Emperor Augustus, found a method of exercising his cruelty by means of this animal [the muraena], for he caused such slaves as had been condemned by him, to be thrown into preserves filled with muraenae; not that the land animals would not have fully sufficed for this purpose, but because he could not see a man so aptly torn to pieces all at once by any other kind of animal.
. This wretched man was originally a freedman, and though he was on one occasion punished by Augustus for his cruelty, he left him a great part of his property. He died B. C. 15. He is supposed to be the same person as the one against whom Augustus wrote some Fescennine verses, mentioned by Macrobius, Sat. B. ii. c. 4.
Tertullian, De Pallio, chapter 5:
6. Equally do I plunge the scalpel into the inhumanity which led Vedius Pollio to expose slaves to fill the bellies of sea-eels. Delighted, forsooth, with his novel savagery, he kept land-monsters, toothless, clawless, hornless: it was his pleasure to turn perforce into wild beasts his fish, which (of course) were to be forthwith cooked, that in their entrails he himself withal might taste some savour of the bodies of his own slaves.
It is not suggested anywhere, note that Vedius Pollio committed any crime in law; merely that he had acted in a gauche and ignoble manner. Seneca states above:
Servis ad statuam licet confugere; cum in servum omnia liceant, est aliquid, quod in hominem licere commune ius animantium vetet.
Slaves are allowed to run and take sanctuary at the statue of a god; though the laws allows a slave to be ill-treated to any extent, there are nevertheless some things, which the common laws of life forbid us to do to a human being.
Literally: “although all things are allowed [to be done] to a slave”.
It is perhaps fortunate that such “law” is not part of our inheritance from Rome.