I remember going to the Killing Fields. Read that after every rain remains float up, and they have collection points where people can leave them. Don't get me started on 'the tree'. One of those places where you can reflect on the evil that humans are capable, and remember that we are not immune from it.
I have accidentally ended up at S-21 and the Killing Caves in Battambang, and the one that struck me was the amount of white western girls breaking down in tears. My Nigerian friend actually pointed it out, and was quite stricken with laughter. For the most part, 99% of Cambodians within S-21 were very casual, talking on the phone and eating ice cream, laughing, etc.
It was just way too distracting to see a bunch of Australian and Californian girls crying everywhere, and even the Khmers comment on this behavior. I found it way more interesting to watch the Khmers watch the foreigners have cathartic experiences. But to put it into an American perspective, when we go visit historic American sites, whether full of tragedy or not, we Americans generally don't take the visit as seriously as a foreign tourist would.
But yes, back to your point, it is good to have an in-your-face reminder of the evil every single human being is capable of inflicting on others.