The first draft of the Convention included political killings, but these provisions were removed in a political and diplomatic compromise following objections from some countries, including the USSR, a permanent security council member. The USSR argued that the Convention's definition should follow the etymology of the term, and may have feared greater international scrutiny of its own Great Purge. Other nations feared that including political groups in the definition would invite international intervention in domestic politics. However leading genocide scholar William Schabas states: “Rigorous examination of the travaux fails to confirm a popular impression in the literature that the opposition to inclusion of political genocide was some Soviet machination. The Soviet views were also shared by a number of other States for whom it is difficult to establish any geographic or social common denominator: Lebanon, Sweden, Brazil, Peru, Venezuela, the Philippines, the Dominican Republic, Iran, Egypt, Belgium, and Uruguay. The exclusion of political groups was in fact originally promoted by a non-governmental organization, the World Jewish Congress, and it corresponded to Raphael Lemkin’s vision of the nature of the crime of genocide.” 
What atrocity motivated a self-serving USSR to object to the provision of political killings drafted into the Convention? (If the question is unanswerable, say "unanswerable")
its own Great Purge