IIRC it was indicated as the motivation behind convicted spy Christopher John Boyce in the movie The Falcon and the Snowman.

https://en.wikipedia.org/wiki/The_Falcon_and_the_Snowman

Well, this is who I was thinking of. Apparently the motivation is disputed though.

https://en.wikipedia.org/wiki/Klaus_Fuchs

"In her 2020 book, Atomic Spy: The Dark Lives of Klaus Fuchs, Nancy Thorndike Greenspan concluded that "Fuchs sought 'the betterment of mankind' [when sharing secrets with the Soviets] ... because "his goal became to balance world power and to prevent nuclear blackmail" according to a New York Times review by the conservative historian Ronald Radosh. Radosh wrote that "this was a post facto justification. The reason Fuchs spied was simply that he was a Communist and a true believer in Stalin and the Soviet Union"."