Short answers, pulled from the story.
Lavrentiy Beria was head of the NKVD, the Soviet secret police, from 1938 to 1945, and one of the most powerful figures under Joseph Stalin. He organised mass deportations, supervised the Katyn massacre of 22,000 Polish officers and intelligentsia, oversaw the Soviet atomic bomb project, and directed the Gulag system of forced labour.
The Katyn massacre was the NKVD execution of approximately 22,000 Polish prisoners of war, military officers, intelligentsia, doctors, and priests in 1940. On the 5th of March 1940, Beria sent Stalin a note recommending their execution as enemies of the Soviet Union; Stalin approved, and the NKVD carried out the killings.
Beria was executed on the 23rd of December 1953. He was tried by a special session of the Supreme Court of the Soviet Union with no defence counsel and no right of appeal, found guilty of treason and other charges, and shot through the forehead by General Pavel Batitsky. His body was cremated and buried in Communal Grave No. 3 at Donskoye Cemetery in Moscow.
In the roughly three months between Stalin's death on the 5th of March 1953 and his arrest on the 26th of June 1953, Beria freed the victims of the Doctors' Plot, promulgated an amnesty releasing approximately 1,200,000 Gulag inmates, returned the Gulag to the Ministry of Justice, scrapped costly construction projects, and advocated greater autonomy for non-Russian nationalities and international détente with the West.
From December 1944, Beria supervised the Soviet atomic bomb project, which Stalin designated 'Task No. 1'. At least 330,000 people, including 10,000 technicians, were involved, and the Gulag provided tens of thousands of workers for uranium mines and processing plants. The project's first nuclear device was completed by the 29th of August 1949.
Khrushchev orchestrated an ambush at a Presidium meeting on the 26th of June 1953, suddenly accusing Beria of being a spy for British intelligence. When Beria appealed to Malenkov, Malenkov pressed a pre-arranged signal button, and Marshal Georgy Zhukov led armed officers into the room to arrest him. Beria was smuggled from the Kremlin in the trunk of a car and held at the Moscow Military District bunker.