I found it quite difficult to connect to the events that happened in that place, despite being surrounded by reminders and guided by an audio-guide with first-hand accounts of the atrocities. It may have been the well manicured lawns and other tourist moping about.
It took some time, some effort, to fully understand what happened. One cannot simply walk into the, or any, genocide museum and comprehend. The audio-guides helped. Everyone had their own little world, that eventually grew and combined with the era 1975-1978, where almost 9000 people where killed in this place alone. It allowed you to absorbed the whole situation bit-by-bit, as it occurred.
Overall, it was estimated that one-in-four people were killed in Cambodia, and by Cambodians. Imagine if one-in-four people you knew were killed.