Not going to dispute this other than to say that it's "the gravest crime against humanity in MODERN TIMES."
In past times, enslaving the populations of entire conquered nations or villages was common. Bringing slaves back to Rome was a regular part of an Army's return. Enslaving one's neighbors has been extremely common across the globe, since the beginning of humanity.
Beyond slavery, there have been marauders like the Huns or the Khans, who would attack a city, and kill every single living thing, and then move on the the next one.
Unfortunately, there are lots of candidates for the award.
Not just Rome, but Egypt, and every other nation, or whatever they were back then.