The French Centre d'Etudes et de Recherches Internationales, along with the French research institution, CNRS and Sciences-Po, have begun an Online Encyclopedia of Mass Violence with the help of the Hamburg Institute for Social Research. The project is under the direction of Jacques Sémelin, whose 2005 book on genocide (which I have but have yet to read) has recently been translated into English and published by Columbia.
The site's still pretty bare bones for the moment, but it's designed to provide information of mass violence based chronologically and geographically, so when it's done, you'll be able to click on any country you want to get information about mass violence in that country. There's also an encyclopedia of terms that looks to be pretty complete.
Strangely enough, for a French initiative, it's only available in English for the moment. The international advisory board includes scholars like Omer Bartov, Samantha Power, Frank Chalk, Antonio Cassesse, Ben Kiernen, René Lemarchand, William Schabas and Eric Weitz, just to name a few.