Interesting thread, I often wonder why I visit such places, and why I avidly read books on these subjects.
Having just come back from visiting the killing fields in Cambodia, I've come to the conclusion that we visit to get a kind of history/psychology lesson (although a very morbid one admittedly).
I think that anyone interested in human nature would find it hard to work out why these horrific events happened, especially the recent genocides in Rwanda and Cambodia.By visiting these places wrongly or rightly we can maybe trying to work out where we have gone wrong as a human race.
Incidentally,I don't think that many people who visit these places are sick and twisted,I was quite humbled by the reverence that was shown by 3 large groups of stag party's who visited Auchvitz the same time as myself, they were obviously not just there to visit the memorial,but there were tears in many of their eyes. These places can teach us so much!