We're basing this off of the percentage of people who died. Bonus points if they die in a horrendous manner
I suspect contestants include Tamerlane, the Khmer Rouge, Congo Free State, and
Zhang Xianzhong (assuming the things they say about him, like the Seven Kill Stele, are not exaggerations made by his enemies). Anybody I forgot?
I think that a more specific definition is needed. Are we going by scale of atrocity, time and breadth of the atrocity committed, if it was committed to the widest group or largest percentage of the population, number killed or number affected, was it deliberate state policy or just a byproduct of national goals
Not to mention how to define subjects.
For example after Nazi-Germany annexed Poland, did that mean that the Poles now were 'subjects' and all atrocities the Germans committed against them fulfil the criteria?
Finally, I believe Sparta certainly deserves a mention. Excluding territories part of larger states, no country ever had such a large part of its population consisting out of slaves. And the things written about Sparta's treatment of the Helots (like the Cryptia) suggest they were treated even worse than usual for slaves.