There is a tradition in partisan history called Blackbooking where an author or author tries to tally the number of deaths inflicted by a particular regime or ideology. Unfortunately, these also tend to turn into "biased slugfest[s]" as most wouldn't set out to do the immense work required to create such a volume without an axe to grind. Still, if read with a healthy portion of salt and a keen eye for their sources, these books can give a fairly good idea of the death tolls and, probably more interestingly, the ways people die under various ideologies. When looking at communism, Courtois, Werth and Paczkowski's 1997 "Black Book of Communism" is your best bet, but is quite controversial. On the other hand, the 1998 reaction "The Black Book of Capitalism" by Perrault et al. is formated more as a rebuttal to the previously mentioned work, with the authors focusing on their own individual gripes, as well as attempting to ascribe deaths not only by direct capitalist action (such as the Chiquita Banana Massacre) but also deaths from more dubious, indirect sources to capitalism. Both books make for interesting critiques of their subject ideologies, but end up being to a greater or lesser degree theatrical in their presentations. The reality is that there is no simple "truth" because tallying the long term deaths inflicted by a political system is always going to rely on speculation (i.e. of the victims of Mao's "Great Leap Forward" who died as a direct result and who would have died anyways because they were independently unhealthy). Hope this helps.