mortality/(time x population)
For example, the Nazis killed 11M over 12 years and the German population was about 70M in 1939. Thus, their deadliness is 11/(70x12) ~ 1/70
The USSR killed 40M (a middling estimate) over about 70 years and the Soviet population was 170M in 1939, so their deadliness is 40/(170x80) ~ 1/340 or about 1/5 as deadly as the Nazis.
This intuitively makes sense to me since the Nazis actively sought to exterminate people, whereas most (not ALL) deaths in the USSR were accidental because communism is useless.