Politicians - at least, successful ones - are easily malleable; while there are undoubtedly some good, reliable politicians around, to truly succeed in this profession, one must be callous, cold, calculating, and selfish.
I know several politicians personally, at varying levels of government, and sudguce it to say that, yes - I believe it is that simple.
They all start out with sunshine, lollipops, and good intentions in their eyes, but soon they lose their way, either because:
(1) they go into self-preservation mode, to save their own careers; or
(2) they get pressured by party members into changing.
Call it a simplistic view oid you wish, but I've met too many politicians at council, State, and Federal level, who have all behaved in the same way, to see it any differently.
The good eggs rarely last.