Oppenheimer, a determined Leftist and almost certainly a communist and source for Soviet spies, regarded the bomb as justified if used against the Nazis but not if used against Japan. The distinguishing aspect was that use against of the A bomb the Nazis would help Stalin and the Soviets but impede their chances to control Japan if used to prompt an earlier surrender. And, as Truman knew, A bombing Japan saved hundreds of thousands of American lives.
As they had Oppenheimer under strictest surveillance and surely were reading all his mail and screening his calls BEFORE the project I can't see where you're getting your information.
That WAS an era where too many were blacklisted being in the wrong place at the wrong time when too many in high government positions actually spying for the Soviets lived, got traitor money and died never being found out.