Just got back from my latest trip to Cambodia. If it hasn't been mentioned already, the infamous S21 is a must visit. Be prepared to be humbled. I had the honour of meeting one of only 12 (known) survivors of the camp, an elderly gentleman called Chum Mey. He could see how visibly upset I was and gave me the warmest of hugs. This wonderful gentleman is a reminder of something that isn't widely talked about in the west these days, and it happened in a lot of our lifetimes. 25% of the population wiped out in a little over 3 years. A reminder of just how brutal humanity can be.
Oh yes, I met him too and bought a copy of his book. He was describing how they used to torture him, often using electrocution. He seemed genuinely pleased to meet tourists who wanted to hear his story.