In Phnom Penh, there is a Khmer Rouge prison, now a museum called Tuol Sleng, or S-21. It has become a must-see for tourists, in the way that Dachau is if you visit Munich. As travelers, I believe it is important to bear witness to this, even if it's difficult. It helps us understand the society and culture a little better. It is also a stark reminder that mass genocide can be committed by any society, not just "evil people."
S-21 is in the center of Phnom Penh. It's an old high school that was converted into a prison and torture center. Duch, now being tried by the ICC, was at the helm. 17,000+ people died at S21 and only 12 survived.