I assume you mean the spies/moles (leakers go to the press, spies/moles go to other countries). I've written a bit about what we know about their motivations here. The short of it is, almost all of the Manhattan Project "moles" were motivated by ideology and/or ego. As opposed to money or blackmail, which are two other traditional motivations. Many of them did genuinely admire Stalin and the USSR: in their mind, the USSR was the country that was doing the most to fight the Nazis (which was arguably true, depending on how you look at it), make a better and more just world, and so on. They considered the anti-Stalin and anti-Communist line to be propaganda. Some of them later claimed to believe that the world would be more stable if more than one country had nuclear weapons, as well.