This is exactly what I'm ,looking to find out. I think there is a distinction between the legal interpretation of genocide: where the acts are committed with the intent to kill the group and the mass killings of members of an ethnic group, which in common language we refer to as a genocide.

From a legal perspective, such mass killings of the Khmer Rouge did not amount to the crime of genocide, as most of their victims were ethnic Khmers targeted for political reasons, as opposed to ethnic, racial, religious or national reasons. However, in practical terms the word genocide refers to mass killings of a national, ethnic, racial and/or religious group. Within this definition, the Khmer Rouge killings of other ethnic Khmers would qualify.

I would love to be corrected if I am wrong.