Human sacrifice was a characteristic of numerous Mediterranean societies in the period of Roman kings and earlier. The Etruscans were known for human sacrifice and mass killing of POWs, connected to a particular penchant they had for public display of disembodied heads (Eckstein 124-5). Various Celtic societies in this period also practised headhunting and human sacrifice (131-2). Livy claims that human sacrifice was a Greek ritual too (22.57), and we know the Romans accused the Carthaginians of quite a lot of it, especially with children, though it appears most of this was propaganda (Hoyos 100-5, we've found large numbers of urns filled with children's bones in the ruins of Ancient Carthage, but this may be associated with normal mortality rather than deliberate sacrifice).

To be honest, it was probably fairly common in many early human societies, the Shang practised it in China (Lynn 34-5), we obviously know the Aztecs did a fair bit of it, and there's even evidence of human sacrifice or cannibalism or some sort of similar thing (perhaps related as much to burial practises as much as religion, not sure) in pre-historic societies.

The Romans probably practised it too in their earliest periods, but it was uncommon by the Mid-Republic and one of the last recorded instances we have of it is the sacrifice of several slaves as a desperate attempt to appease the gods in the aftermath of Cannae (Livy 22.57). Livy specifically points out that these sacrifices were "wholly alien to the Roman spirit". It was no doubt an ancient practise that was resurrected out of desperate circumstances. It should be noted of course that the Romans continued to bury Vestal Virgins alive if found to have had sex while serving as Vestals, which is surely a form of human sacrifice.

The sacrifice of Vestal Virgins and human sacrifice may have been connected. Similar sacrifices, and the sacrifice of Vestal Virgins, had occurred 228 BC in the face of a huge Gallic invasion, and again in 113 BC related to the massive migration of Germans floating around at about this time (Beard 180). We don't know the connection or details of these sacrifices (other than the specific selection of victims, a pair of Gauls and a pair of Greeks apparently), but it seems clear they were fairly desperate, atypical rituals.

As time went on the Romans, as expressed by Livy with his "wholly alien" remark (written well after the last recorded instance of the above sacrifice occurred) took a pretty dim view of human sacrifice. Their criticism of Carthage for their widespread use of the practice aside, the Romans also used it as a justification for the suppression of religious groups such as the Druids (489), and it continued to be used as a piece of propaganda against barbarians such as regarding the Germans in the aftermath of Teutoberg Forest in AD 9 (512). By the Imperial period, it's pretty clear the Romans didn't think much of human sacrifice at all.

I have never heard of human sacrifice being associated with Saturnalia, that does not really sound possible to me considering the rarity of the practise among the Romans even in the Mid Republic, and the even harsher attitudes we find towards it well before the Imperial period. I'd have to see what sources you have suggest human sacrifices occured at Saturnalia, but as far as I know such a thing did not occur.

Sources:

Beard, Mary. SPQR, London: Profile Books, 2015.

Eckstein, Arthur M., Mediterranean Anarchy, Interstate War, and the Rise of Rome. Los Angeles, University of California Press, 2006.

Hoyos, Dexter. The Carthaginians. London: Routledge, 2010.

Lynn, John A. Battle: A History of Combat and Culture, From Ancient Greece to Modern America, Westview Press, 2003.

What exactly are your sources suggesting human sacrifice?