His name was Theodore (Ted) Alvin Hall (changed from “Holtzman” to escape American anti-Semitism, by the way).
From Wikipedia:
Hall later claimed that as it became clear in the summer of 1944 that Germany was losing the war and would not ever manage to develop an atomic bomb, he became concerned about the consequences of an American monopoly on atomic weapons once the war ended. He was especially worried about the possibility of the emergence of a fascist government in the United States, should it have such a nuclear monopoly and want to keep it that way.
…
In a written statement published in 1997, Hall came very close to admitting that the Soviet spy cable identifying him as a Soviet asset was accurate, although obliquely, saying that in the immediate postwar years, he felt strongly that “an American monopoly” on nuclear weapons “was dangerous and should be avoided:”
To help prevent that monopoly I contemplated a brief encounter with a Soviet agent, just to inform them of the existence of the A-bomb project. I anticipated a very limited contact. With any luck, it might easily have turned out that way, but it was not to be.
A year before his death, he gave a more direct confession in an interview for the TV-series Cold War on CNN in 1998, saying:
I decided to give atomic secrets to the Russians because it seemed to me that it was important that there should be no monopoly, which could turn one nation into a menace and turn it loose on the world as … Nazi Germany developed. There seemed to be only one answer to what one should do. The right thing to do was to act to break the American monopoly.
From The Guardian:
Hall had earlier told the authors of the book Bombshell: “Maybe the course of history, if unchanged, could have led to atomic war in the past 50 years; for example, the bomb might have been dropped on China in 1949 or the early 1950s. Well, if I helped to prevent that, I accept the charge.”
Well said!
Of course, he never should have helped create The Bomb in the first place!