Numerous authors (Pliny, Seneca, Cassius Dio) wrote of Vedius Pollio, a man who was said to feed his slaves to murenae, which were eels or lampreys of some kind. The story may have been exaggerated to some degree, as horror stories often are. The point is that all of the authors who wrote on Vedius Pollio all thought the practice was absolutely cruel and psychotic:

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. 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. (Seneca, On Clemency, 1.18)

In his Roman History, Cassius Dio even tells the story of Augustus's reaction to such a killing:

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. 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,' and when they were brought, he ordered them to be broken." (Dio, Roman History, 54.23)

The fact that everyone writing about Vedius Pollio goes out of their way to say how barbaric a punishment this was indicates that this practice was far from normal in Roman society.