Another example: the origin of the word "decimate". "Decimation" was a Roman military punishment to military units in provinces that rebel against Rome. I don't believe it was widespread but it was common enough to spawn the modern word. "Deci-" here is Latin meaning "10". The punishment was this: soldiers (typically) were put in groups of 10. Each group was responsible for picking which of those 10 would die and be responsible for killing that person. The general way Rome quashed rebellion in general was brutal.

The amazing thing we actually have first hand accounts on Rome's military conquests in the forms of the writings of Julius Caesar (eg with the conquest of Gaul, which was essentially genocide). Now first hand accounts aren't necessarily accurate (eg Herodotus tended to embellish) but there are other accounts that lend a lot of credence to Caesar's accounts.