As a percentage of the total world population:
If one looks superficially, then war in the 20th century killed approximately double the people than war in the 19th, which is approximately double or more that in the 18th.
Deaths from natural catastrophe (outside of famine) has decreased since the 19th century when individual earthquakes killed people by the near millions in their shoddy built homes. However the raw amount of death is much less significant than that from war or the next major causes.
Death from preventible disease has clearly gone down since the late 1700s because of vaccines, starting with Smallpox, and progressing through today. Plagues have been a major problem throughout history but are decreasing over time because of the use of the scientific method in combatting such plagues. Death through preventible disease would be much larger than wars or natural disasters combined, particularly if one considers infant mortality. The effect of disease in the New World exterminated between 80 and 90% of the indigineous population between 1492 and 1800. This alone could account for nearly 100 million deaths - double the count of WWII.
Another form of death in history, which would nearly equal that of war, would be due to deliberate genocide. While there are clear cases of large scale genocide in the 20th century from Germany, Russia, China, and Turkey there are also potentially even larger examples due to colonization and the slave trade. Estimates from deaths due to the slave trade between 1492 and 1883 have been as high as 100 million, but at least 20 million would be a very conservative estimate. Death due to genocide could be considered part of death due to war.
All of these causes of death pale in significance to deaths due to famine, which prior to modern agricultural methods due to invention and later formal science would outnumber all other causes combined until the 20th century. The amount of death due to famine would also have generally decreased over the last 500 years, particularly if one considers accounts from China and India. Famine could of course be considered a part of catastrophe.
Therefore Schrafinator's assertion that death due to disease, catastrophe, and war has decreased over the last several centuries, if taken as a whole rather than picked apart for each individual cause, is essentially correct, especially if viewed as a percentage of the total population of the Earth rather than as a raw number.
I naturally feel no need to agree with those who assert the end times are near because the amount of deaths due to various natural and man-made calamities are increasing.
This reply, while to Iano, represents a general evaluation of Schrafinator's assertion. Iano {ABE - and Omnivorous were} correct in implying more people were killed in the 20th century due to war than in previous centuries, although the religious wars between Protestants and Catholics in the 17th century would have come in second at least if one considers the last 500 years and as a percentage of the world population. The prior deprivations of the Mongol Empire, the fall of the Western Roman Empire, and various convulsions in other areas, particularly China, may have even exceeded this figure as a percentage of world population but the further one goes back in time the more potential error is introduced.
Edited by anglagard, : To include omnivorous.