A really thorough ethnic cleansing is generally more difficult without modern government structures and mass media to really gin up the hate: in Rwanda, ordinary Hutu slaughtered half a million Tutsi in three months -- 2/3 of the entire Tutsi population at the time -- and they did it with garden tools[1].

That kind of death used to take a standing army backed by decades, if not centuries, of training and discipline: Caesar's slaughter of the Helvetii is the first that comes to mind, maybe followed by the Qin wiping out the Song during China's Warring States period.

[1] https://en.wikipedia.org/wiki/Rwandan_genocide