The second-biggest cause of death in the 20th century was democide — where a government deliberately kills its own citizens — either officially or unofficially. In addition to mass shootings, etc., democide also includes deaths in concentration camps, forced labor and deliberate famines, as the Russians forced on the Ukrainians in the 1930s. Democide goes beyond genocide, which is the murder of people due to their national, ethnic, racial or religious group membership. Democide also includes the murder of political opponents.
Of the estimated 260 million people killed through democide from 1900 to 1999, the Chinese communists accounted for more than 80 million, the Soviet communists for 63 million and the German national socialists (Nazis) 21 million. Other socialist regimes, including Cambodia, Vietnam, Poland, Yugoslavia, Cuba and North Korea, accounted for perhaps another 8 million deaths. (Note: All of the above numbers are obviously imprecise with a large error range.) The rest of the deaths were due to the actions of various authoritarian regimes and colonial rulers, often in what became socialist countries. Extra-judicial killings by free-market democratic governments of their own citizens are rare.
Socialism comes in various forms. The most extreme, known as communism, was that practiced by the Soviet Union and China before the reforms in the early 1990s and the late 1970s, respectively. Milder versions were attempted in many countries where typically major industries and large enterprises were taken over or controlled by the state. Small shops, restaurants and small service businesses were still allowed to be private. There are fatal flaws with any form of socialism. Rather than the market determining prices, bureaucrats set prices, which eventually leads to a misallocation of resources; hence, slower growth. Socialism, by its very nature, relies on coercion, and as people work their way around the coercive state, black markets expand, which requires more coercion to control and on, and on.