So you are under the assumption that there is someone who is not corrupt? Money makes the world go round, influence can get you anything, sometimes its cheap, sometimes expensive, you just have to name the right price.
People sell their souls for money, selling their responsibility towards their country won't be that tough...
I'm not saying the CIA can't become corrupt.. only that b/c they are so secretive, the American citizens would not even know if any CIA members did anything against the law. Because of this secrecy, it is pointless to suggest their corruptness... we can only hope that they are still on our side ;)