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...
[0] https://www.amazon.com/Double-Cross-Story-D-Day-Spies/dp/030...
To be fair tough, Fuchs made important enough contributions that the Manhattan project might well have been too late to bomb japan before the war ended.
https://en.m.wikipedia.org/wiki/Motives_for_spying
In the case of the early Los Alamos scientists: Ideology
IIRC, in the 30s and 40s it was a lot more common and respectable to be a Communist due to the experience of the Great Depression an limited information about the failures of central planning.
When said governments are very clearly acting against their own people, is it no surprise that defectors may choose to side with an opposing ideology?
Plans were drawn up and advocates were seriously proposing to enact them: https://www.businessinsider.com/the-pentagon-estimated-204-a...