What was exceptional in the genocides and mass murders of the 20th
century was not their scale - after all, the new industrial society gave
ability to kill people more efficiently - but that they were (when given
publicity) condemned. Large scale atrocities were not uncommon
earlier, but they were not usually condemned: Nobody claimed that
the leaders of Argentine should have been brought to justice even
when they managed one the most efficient genocides of the 19th
century when they practically wiped out all the native peoples
of Argentine. To this day, only 30 000 of them remain.

So, the genocides of the 20th century were no exceptions in the
peaceful flow of human history; that stream has always been red
from human blood, and what was exceptional in the 20th century
was that the atrocities were almost universally loathed and some of
their leaders and "ordinary" participants were brought to justice.