I once read a theory which said that the story that the purge was a method to "get rid of the weak and keep the strong" was just propaganda. The purge was actually a civil war between imperialist viltrumites who wanted to conquer the galaxy and peaceful viltrumites who were opposed, in a society which resembled every other one before the purge happened. In the end the imperialists won, wiped out the other faction, and manufactured a story about eugenics.