(BACKGROUND INFO) In Chapter "Terror" in "Khmer Rouge rule of Cambodia" in Wikipedia

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The Khmer Rouge government arrested, tortured and eventually executed anyone suspected of belonging to several categories of supposed "enemies":

Anyone with connections to the former government or with foreign governments.

Professionals and intellectuals ---- in practice this included almost everyone with an education, people who understood a foreign language and even people who required glasses. Ironically and hypocritically, Pol Pot (led the Khmer Rouge from 1963 until 1997) himself was a university-educated man (albeit a drop-out) with a taste for French literature and was also a fluent French speaker. Many artists, including musicians, writers and film makers were executed. Some like Ros Sereysothea, Pan Ron and Sinn Sisamouth gained posthumous fame for their talents and are still popular with Khmers today.

Ethic Vietnamese, ethnic Chinese, ethnic Thai and other minorities in Eastern Highland, Cambodian Christians (most of whom were Catholic, and the Catholic Church in general), Muslims and the Buddhist monks.

"Economic saboteurs:" many of the former urban dwellers (who had not starved to death in the first place) were deemed to be guilty by virtue of their lack of agricultural ability.
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