“re your
“Europe never had the massive executions and sacrifices that happened in South America.”
One would tend to disagree with that apparently unsupported assumption. Read Caesar`s and Roman histories wherein they wiped out entire Gaulic and Germanic tribes, men, women and children by the hundreds of thousands, executions & massacres.
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.