Agrarian Reform, otherwise known as "Communism", has killed six times more people than the Doomsday Flu of 1918. Nothing has killed more people in all of human history.
Doesn't stop people trying it though. It's a memetic disease and some people have no immunity.
Actually I've seen an argument that environmentalism has killed more people than communism--mostly excess malaria deaths due to the banning of DDT.
I suspect it's not true, as the person presenting it took the low-end estimates for communist murders (terror famines and mismanagment-induced famines included) in all cases except Cambodia.