The linked article on Ted Hall has him explaining why he did it. All such justifications must be taken with a grain of salt, of course, but he claims to have feared that if a post-war depression ensued, the USA could turn totalitarian and fascist, in which case it would be better if the USA did not have a monopoly in nuclear weapons.

He also recounts the extent that his views were shaped by his slightly older roommates at Harvard, whom he was obviously in awe of.

Rather more self-servingly, he claims that he probably would not have spied if he had known how despotic the Soviet Union was.

Money was apparently not involved.

https://www.nytimes.com/1997/09/14/magazine/the-boy-who-gave...