I haven't read any Roman history since I took five years of Latin in the 1960s.
However, I do not recall that mass slaughter was a common practice unless there was open and violent rebellion.
In addition, many decades of relative tranquility would typically follow periods of great bloodshed.
To my eye, that kind of military reprisal was common throughout history in every corner of the world.
As to ongoing religious human sacrifice, once again I am no expert, but my impression is that it was never common in Europe or European colonies.
Cassell’s Biographical Dictionary, etc. [With plates. Edited by T. T. Shore.]
p426
“In 55 BC Caesar drove back with great slaughter some other German tribes who sought to cross the Rhine”
Caesar sent out forces under Trebonuius and :Labienus to slaughter the Manipii and the Aduatuci.”
Caesar 53-52 B.C.
By Nigel Cawthorne
p.xv “multitudes of the inhabitants were put to the sword”
gaius iulius caesar gallic war, Book 1
By C. Julius Caesar
“tens of thousands were cut down, as they fled, including many women and children”.
Julius Caesar
By Philip Freeman p.177
footnote 4.15d “”including the massacre of women and children”
/ p.177 The Landmark Julius Caesar: The Complete Works : Gallic War, Civil War ...