Russian Lavrenti (also spelled Lavrenty) Beria was born in 1899. At age 18, he participated in the Bolshevik Revolution and subsequently worked for a number of governmental agencies whose purpose was to prevent any counter-revolution from occurring. Beria's work in such agencies continued and by 1938 he was in Moscow and would soon become head of the Communist's secret police force.
Beria did well for himself as long as Joseph Stalin was in power, but when Stalin died in 1953 (perhaps poisoned by Beria) and Beria tried to take his place as the Soviet leader, he found stiff opposition in the form of Nikita Khrushchev and others. Ultimately, Beria, whose work with the secret police had been responsible for eliminating so many other political opponents, was himself executed for treasonous activites two days before Christmas in 1953.
Not only was Beria responsible for the deaths of thousands of people, as brettd points out, but also Stalin put Beria in charge of helping the Soviets develop a nuclear weapon. Beria-directed spies acquired much of the knowledge they needed to build their bomb from the United States and by 1949 the weapon was tested.
Thus, Beria is important not only as one of the most ruthless and dangerous men in modern Soviet history, but also as the man who orchestrated the rise of Russia as a nuclear power, which caused decades of tension with the United States.