Yeah, these "who is the worst" questions usually end up being a measure of capability as opposed to evil.
Why could Mao be responsible for 30-40 million deaths? Because he was in charge of China. By contrast, the Khmer Rouge had maybe a million victims -- which was like 1/4 of the Cambodian population. So who is worse? Would the Khmer Rouge have killed 30 million if in charge of China? What if Mao were leading Cambodia?
I've long thought that Ariel Sharon and Milosevic had more or less the same view of the world. Sharon was leading Israel, which at the time was still a liberal leaning nation with close ties to the US. Milosevic filled a power vacuum after Tito and had near total control. If they had swapped spots, I suspect their behavior would have been more or less the same.
Trying to run a bracket of "who is the worst person in the world" is a hopeless endeavor, in my view. I think it's better to look at things like this: there is a maximum evilness that a person can achieve, because we simply don't care that much about transgressions beyond that point. Like, if there's an alternate universe in which Hitler was able to fully exterminate the Jews in Europe ALONG WITH the slavic population, is that Hitler worse than real Hitler? The body count would be higher, for sure. But once the victims are in the millions, does it even matter how many -- especially since real Hitler wanted to do those things but just didn't have time.
So on this model, I'd say there are many people at that level of maximum evil. Putin, KJU, the "stan" tyrants, the Sudanese ethnic cleansers, the Houthis, the Saudis, etc.