I think that where we are today, it's more important than ever to really recognise that there is absolutely no "evilest" person. And moreso, almost all of them genuinely believed they were being a force for good.
The main reason for there being no "evilest" is because there are not just tens, or hundreds, but thousands of people who have committed horrific atrocities which even as individual acts would be in contention for the evilest.
Just within the Holocaust, you have so many individuals perpetrating such evil. The croatian fascist groups always stick out in my memory for the raw personal brutality.
However, there are so many comparable situations throughout history that we know very little about. Sacking of cities, mass rapes, Jeffrey fucking epstein, what's been happening in Sudan, what happened in Ethiopia a few years ago, the Holocaust, the Mongols, the ottomans, the British empire, south American dictators, the Japanese empire, the russian empire, the soviet union, al Qaeda, the Taliban, ISIS, the IRA, the Vikings, and on, and on, and on.
Yes, arguments can be made about how much more evil things are when they are organised etc. However, in my mind, organisation is just a tool that we have improved. We need to be hyper vigilant that anyone, at any time can end up being so radicalised that they are willing to perpetrate evil. Including ourselves. We are not really different from these evil humans.