The question is raised on The Volokh Conspiracy, not for the first time. Nobody is saying genocide is OK - but there is an obsessive attempt to draw a line between genocide and "mere" mass murder. If somebody wants to make the point that Stalin was a much nicer guy than Hitler because - despite Stalin maybe* being relatively responsible for more deaths - Stalin's deaths were just murders rather than genocide, I am not going to be especially impressed. Nor, for that matter, am I going to be impressed with somebody who argues that Hitler is nicer than Stalin because his death-count was lower. Both Hitler and Stalin were quite bad enough, and the idea of some sort of contest between them is macabre.
The issue came up because of the book Stalin's Genocides, which argues - well, it is pretty clear from the title what it argues. Apparently, the issue is important from the standpoint of international law because if a country is committing genocide then it is legitimate to interfere, but if a country is only slaughtering millions of its own people without regard to ethnicity (kill all the educated people or all the rich people), then that is an internal matter. It's pretty clear something is a bit off with international law here, but it's hard to get people to accept a definition that makes their own country guilty.
*I've heard figures going both ways.