My guess is and has been that, the carving out of such a large chunk of the Gallic population (mostly enslaved, some press-ganged into auxiliary service in other parts of the Empire) led to a centuries-long, probably fatal crisis as the Roman-era cooling of the climate led to mass migration along the steppe and into Europe.
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.