A university historian complained recently that these days undergraduates only know about two things: Hitler and Stalin. Apparently, everones does them, first for GCSEs, and then again for A levels. So people who are interested enough in history to take a degree in it arrive with no eal knowledge of anything that happened before 1900.
You can see why this happens, though: the Second World War and related matters seems to be the one reliably-popular period, reflected in TV series, like Timewatch, whose remit is broad in both time and space, but chooses WW2 at least every second programme, and indeed entire channels: if you took WW2 and Egypt out of UK History (which would be no bad thing), you'd be left with just the adverts (which would). And there is a steady stream of semi-popular biographies of Hitler, Stalin, and Hitler-and-Stalin, presumably because they are in demand. Not by me, though. The biographical approach to history tends to emphasise narrative at the expense of analysis. Even if one could examine people's motives, this would be unsatisfying, as unsatisfying as the parlour game these biographies become: who was the maddest? most evil? My snap answer would be that Stalin was maddest because he was prepared to act directly contrary to his long-term interests: imprisoning or executing all of he army officers just before a war. I'm not sure about evillest, since it covers both the moral content of your intentions and the acceptability of the means used to carry them out.
There is a much more interesting moral question to be addressed, though. Hitler and Stalin were unique, generating massive suffering. But they didn't do it alone. It is conveneient to see their actions as a one-off (or two-off). This would be wrong. The 20th century is full of genocides, from the Armenians to Pol Pot and Rwanda. Genocide is not an aberration in modern politics - it seems endemic. More important than the issue of what a dictator thought, or did, is the issue of how so many other sane, reasonable, people, helped them. And helped them with gusto. Even if it difficult to stand up to a totalitarian regime and say "no", it is much less difficult to work inefficiently in carrying out its more repellent instructions. Or so you'd think.
There is a genocidal rhetoric which makes it easier for people to do things that they would otherwise find deeply troubling. The key element is the denial of the humanity of the target group: literally 'demonised', 'inhuman', 'bestial'. If they are outside the pale, incurable, not susceptible to reason, all you can do is lock them up, or kill them. Otherwise you will always be threatened.
When the first photos from the Iraqi miltary prison appeared, there was, as well as shock and disgust, a general feeling of surprise: why did they even think of doing it? To me, it wasn't surprising, since it fitted in with the rhetoric of the war on terror, and the treatment of the prisoners at Guantanamo Bay. If you start off by saying a group of people cannot be afforded human rights, then it takes only a small step to say that they are hardly human at all.
There's a pernicious piece of Jesuitry from Lenin : ""Liberty is precious, so precious that it must be rationed." http://encarta.msn.com/media_461577172_761562790_-1_1/Vladimir_Ilich_Lenin_Quick_Facts.html Who is doing the rationing? Do you trust them?