Still breaks down to "Us and Them".Grandma -
This is actually my field of expertise as a journalist, so I'm going to claim I actually know what I am talking about on this one!
Mot experts consider genocide to be a deliberate attempt to wipe a people off the map. So what we consider 'a people' could be defined in several different ways.
But it's usually considered that it is one group (i.e. Germans, Australians) attacking another group (Namibian Herero, Tasmanian Aborigines).
I do not believe Stalin, Mao nor Pol Pot committed genocide, because they did not (by and large) target specific ethnicities, religions or tribes. They committed mass murder.
However, Rwanda did experience genocide (Hutu against Tutsi) in 1994, and there have been several other instances in this century.