This list is pretty modern history (except the jokish entry of GOD), BECAUSE its sure fire science to count casualties in these modern times, why further back its just idle speculation mostly.
We could say for example the Timurids were the Maos of their time, their Khan being responsible for an estimated 30-40m dead more or less. But firstly, not only is virtually hard to confirm that and is just vague guessing game based on rare obscure history documents (estimates can wildly vary basd on the biasm of the sources). But also the expectations and set standard of things were quite different from way back then (depend on how far back), to the more civilized modern times. Back then, it wasnt really so "bad" to going around killing everything in sight, for lack of better words.
They were less likely, to say the least, to scrutinize eachother and name their own Hitlers and set him as the benchmark for evil. On the contrary, usually the most evil massmurdering son of a bitch was completely worshipped, and had the title GREAT added to his name, with every other nation envying them for not having an own massmurdering leader like that

(Not only blaming our ancestors, we are guilty of it too, not just seeing it in historical context, but we nostalgically semi worship the most awesome massmurderers of our history).
I hope the made the distinction clear enough. Its quite essential an
evil list like this is modern, to make sense. Otherwise it would be pointless and counter productive. If we count non modern times, we would be more likely to discuss who is the greatest, not who is the most evil (it would sound odd calling Caesar and Alexander evil, doesnt it? Even if it makes sense with our standards). Not much different between that mostly, except the tides of time.