It is very difficult, and perhaps impossible, to objectively differentiate between different kinds of mass killing. It is very difficult to call one kind of mass killing “genocide” and to argue that it is more horrendous than other types of killing.
For example, let us look at two incidents of mass killing that occurred near to one another in both time and place. These are the Holocaust and the millions of deaths caused by Joseph Stalin. Typically, we say that the Holocaust was genocide and Stalin’s killings were not. Hitler wanted the Jews killed because he believed that they were enemies of the German people and would destroy German civilization if they could. Stalin wanted to kill people for at least two reasons. One the one hand, he wanted the “kulaks” to be killed as a class because he felt that they stood in the way of communism. On the other hand, he wanted to purge large numbers of other people (either killing them outright or allowing them to die in prison camps) because he felt that they threatened his own hold on power. It is very hard to distinguish between these two mass killings on a moral level.
Therefore, it seems more useful to talk about all mass killings together. If we label some as genocide, we diminish the importance of other mass killings and we seem to excuse the people who carried them out.