Genocide haunts the 20th century. The Holocaust, the German attempt to exterminate the Jews, comes immediately to mind -- though similar policies against the Roma, gays, and others are less well known. The genocide in Rwanda, where the world watched the massacre of the Tutsi by the Hutu and did nothing, and the Srebrenica massacre of Bosnian Muslims by Serbs are fresh in our memory.
There was plenty of genocide in the 19th century, and many have been killed in the 21st. The 20th does not stand out.
At least according to one source, the Armenian genocide occurred in several waves, starting in the late 19th c:
The Ottoman Centuries: The Rise and Fall of the Turkish Empire
by John Patrick Douglas Balfour , Lord Kinross