There is a very simple explanation to this. These people may be selfish and crooked, they may even be inveterate villains (like Hitler and the crew), but they have the same trait in common, i.e. lust for power which can be satisfied only through organising people around them. If they are clever at that, they understand that their own ambitions can be fulfilled to the full only through making lives of the people around them better (be it a family, a city, a state, whatever). Even if you are as greedy as Midas, your greed can only be fed up to the limit if you are constantly giving back