Militarism
Alliances
Imperialism
Nationalism
The Red Army was formed and used to keep the Bolsheviks in power.
Lenin immediately took Russia out of the war
A brutal civil war ensued in Russia after the October Rev that lasted beyond the end of WWI.
Stalin was insanely paranoid and as a matter of course regularly killed those near him.
December 1, 1934: Sergei Kirov, the party chief of Leningrad and important member of the Politburo, was murdered. Stalin probably had this done but it can't be known for sure.
1936-1938: Most high ranking Soviet officials were arrested and put on trial. They all confessed to crimes against the party and state. Stalin tortured them and held their families. Millions of common Soviets were arrested and killed or sent to gulags.
When this was done, no opposition of Stalin remained. All that remained was Stalin and whatever he wanted the USSR to be. Few people have ever had this kind of absolute power and he's one of history's greatest monsters.