There appears to be a standard Phnom Phen tourist route that takes you first to "the Killing Fields" and then to the Toul Sleng Genocide Museum, also referred to as the "torture prison" (by some we met in PP). After your trip to the Killing Fields, you may be thinking that you are at rock bottom in terms of your faith in humanity, but the torture prison shocks and moves you further below.
My wife and I paid the extra US$6 (before any tip) for a guide, which I would suggest as being a "must". The rooms at the school-made-prison do a lot of speaking for themselves, but the guide provides a more detailed perspective. For example, during our tour of the facility, our guide shared with us her stories of how her life has been significantly affected by the atrocities of Paul Pot's Khmer Rouge regime, making our experience all the more educational and moving.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.