well if we stick to modern issues, world war 2 would make the black death down on the lists. the number of deaths that occurred during government caused famines in China (great leap) and Russia have estimates that are worse and I would classify them are worse than the Black Death because they were purposeful and fully avoidable.

It is one thing to have a virulent disease wipe out an ignorant people with limited communication and scientific ability to cope. It is wholly another what is done purposefully by governments to their own and others. Japan, China, and Russia, all have been guilty of incredible death tolls because of treatment of their own people and those they conquered. If anything that make the Nazi's look like amateurs but for the most part they get a pass for their past transgressions