It is generally agreed by conventional historians that if famines, prison and labor camp mortality, and state terrorism (deportations and political purges) are taken into account, Stalin and his colleagues were directly or indirectly responsible for the deaths of millions. How many millions died under Stalin is greatly disputed. The 1926 census shows the population of the Soviet Union at 147 million and in 1937 another census found a population of between 162 and 163 million. This was 14 million less than the projected population value and was suppressed as a "wrecker's census" with the census takers severely punished. A census was taken again in 1939, but its published figure of 170 million has been generally attributed directly to the decision of Stalin[13] (see also Demographics of the Soviet Union). Even still, the 1939 census displayed a 3.2 million to 5.5 million excess deaths which leaves the reports of a 1937 census subject to skepticism. Note that the figure of 14 million does not have to imply 14 million additional deaths, since as many as 3 million may be births that never took place due to reduced fertility and choice.Since "the margin of error" with regard to the number of Stalin's victims is virtually impossible to narrow down to a universally accepted figure, various historians have come up with extremely varying estimates of the number of victims, from under 10 to over 50 million deaths.
For the record, that figure is a lot closer to 50 million than 10 million (10 million is the figure generally given by neo-Marxists and Stalinist apologists). And that's just the work of Stalin alone, not counting those killed by the brutal reigimes of Mao, Idi Amin, Pol Pot, Castro / Guevara, or the countless other massacres that happened in Marxist / Communist states during the Cold War.
Through the Cold War there were a number of excellent anti-Communist books written which really captured the Soviet era, and its failings. I'd highly recommend you read Arthur Koestler's 'Darkness at Noon', George Orwell's 'Essays', 'Nineteen Eighty-Four' and 'Animal Farm', and anti-Communist dissident Vaclav Havel's 'Power of the Powerless' if you get a chance, as well as any decent history of the cold war. Friedrich Hayek's 'Road to Serfdom' also has a number of excellent chapters on topics like 'Economic Control and Totalitarianism' which discuss how any left wing economic statism automatically leads to the loss of freedom in all other areas (contrary to the Libertarian 'world's smallest political quiz' myth that some on the left want 'social freedom' without 'economic freedom').
Make no mistake, old Marxism wasn't just a system that 'didn't work'; it was a brutal and violent system that sent undercover communist operatives and set up parties to all corners of the world - all centrally funded from Moscow - with the explicit aim of overthrowing the world's free states. And it's always been a bankrupt and dangerous ideology which deserves to die.