I think you cannot visit Cambodia or Phnom Penh without trying to understand a little bit of it's history. Well, I think you shouldn't. The Tuol Sleng museum in Phnom Penh is one of the places where you can get a glimpse of history.

Tuol Sleng used to be a highschool in a rather affluent suburb of Phnom Penh before the Khmer Rouge decided to turn it into a high security prison and extermination camp. People were brought here to ascertain their guilt. That they could be innocent was never a possibility since the Angkar, the party, doesn't make mistakes. After the interrogators - some of them as young as 13 - got their confessions the prisoners were killed and burried on the so called Killing fields "Cheung Ek". 17,000 people were killed this way. When the Vietnamese army arrived in PP in 1979, they only found seven survivors.

If you go to Tuol Sleng - and I think you should - make sure you arrive there shortly before 10 am. Because this is when they show an approx. two hours long movie in which former guards and two of the survivors talk about their experiences and confront each other. Personally I found the movie to be much more impressive then the museum. The museum itself is certainly informative, but in some way I think it is "sanitized". I has been tidied up for the visitors.
The movie on the other hand is very pesonal and very emotional. So I'd definitely recommend that you go and see it.