Who is the largest mass murderer in history?

The problem with identifying the largest mass murderer in history is that there are incidents of mass murder which have not been accurately recorded or counted, and there are ambiguous situations as well; the policies of Chairman Mao of China lead to the death by starvation of some forty million Chinese, but was that murder or just negligent manslaughter? We can't be sure. But to answer your question, it was probably Adolph Hitler. Other candidates include Chairman Mao, Josef Stalin, and Hideki Tojo.