Did Obama actually said that? I don't know if it's true, but I heard that Obama told Bernie that he can't be the good guy and be president.

Corruption aside, once getting into position of political power, it is actually harder to maintain it because of different stakeholders involved. Take the military industrial complex, for example. We rightly chastise its profit taking motive by artificially inducing wars. But the sad truth is that they are jobs providers to peripheral places where there are little to no opportunities for many, especially in the desert states in the American Southwest and in isolated regions of Scotland and Northern England in the case of UK. No politicians want to be branded as jobs destroyer.

Even without the opportunity for personal corruption, balancing genuine concerns and interests is a hard juggling act for any politicians. And that's not even including campaign financing and its trap of being beholden to the donor. Unfortunately, there is strong correlation between electoral success and how much money is thrown into the campaigning. It is one factor as to why the Citizens United was allowed by the US Supreme Court.