As with most other toplists, there is no conclusive answer to this (though the thread starter already knows this.. but many others here don't seem to). Imagine for a moment that some of the mentioned serial killers would have gained dictatorship of a country along with all the power and army resources that goes with it. He would very quickly get into the really large scale systematic murder (millions of people). Just because they didn't happen to have the possibility for that doesn't really make them any less worse than those that
did have power, backing and circumstances to enable going through with it.
Furthermore, give any "good" person too much power and he will tend towards atrocities. The line between a normal "good guy" and the worst kind of shit-eating, ebola-spewing rapacious motherfucking bastard has started to look somewhat thin to me after observing the human nature for a time. Most people have the capacity and willingness to torture, rape and kill given the opportunity, correct circumstances and enough time for them to develop a callous attitude. What's more important, most people have the cowardice, spinelessness and apathy to just let things spiral out of control without doing anything about it.
"When bad men combine, the good must associate; else they will fall one by one, an unpitied sacrifice in a contemptible struggle."
-Edmund Burke (attributed to Alexis de Tocqueville as well)
Though perhaps the most important aspects allowing for large scale atrocities is that people bullshit themselves into thinking that they're actually not doing anything wrong. They buy into any kind of dogma, superstition or a word from an "authority" that "makes it all right" and extremely violently resist any sort of opinion to the contrary, mentally and physically. Individual dictators wouldn't have become as bad as they have if they wouldn't have had the power base to do it.
Sorry fellas - I don't think there is any validity in comparing "who was worst".
In a way, Stalin was right - "The death of one man is a tragedy. The death of a million is a statistic".
How do we define which is "worse" ?
Is a rapist worse than a murderer? Or are they both equally bad?
Roughneck wrote words of wisdom. No wonder.. unlike the vast majority of people here, he has actually experienced war and apparently many of the worst kinds of things that go with it. I certainly haven't.. and I hope that I never will.