Man's Darkest Hour... A Genocide?

It was mentioned in class that the Rwandan Genocide was the most concentrated act of human killing in history.  While that may be true, the most “effective” act of human killing was in Cambodia under the Khmer Rouge.  Some of you might remember having heard about Cambodia in US History, in how Vietnam invaded them after the Vietnam War.  Anyhow, the Khmer Rouge led a massive campaign of economic centralization, with the intent of creating a society where everyone participated in laboring on large, collective farms.  In the three years the Khmers held power (from 1975 to 1978), anywhere from 1 million to 3.3 million people were killed.  I believe 2 million is the number most people believe, but given the methods used to eliminate the victims (see the Killing Fields) we can never really know for sure.  Furthermore, it is important to keep in mind that Cambodia’s population before the Khmers took power was around 7 million, meaning anywhere from 20% to 47% of the country’s population was killed in just 3 years time.  As horrific as the whole ordeal was, the main reason I wish to bring it up is because it was arguably a genocide.  While the Khmer Rouge mainly targeted intellectuals and professionals (as well anyone who opposed their plan), they also killed off anyone who was religious as well anyone of non-Khmer blood (Chinese, Thai, Vietnamese, etc.).  The Khmer Rouge used the reasoning that these victims were, by being of another gene pool or of any religious group, inherently enemies of the state.  So, does that mean those victims were directly killed because of political reasons (as opposed to ethnic/religious reasons), even if those political reasons are openly tied to said ethnic/religious reasons?  I realize that, at this point, all we are doing is determining what circle of hell the perpetrators end up in.  Nonetheless, I do think the whole ordeal is a good way to measure our understanding of the definition of genocide.