I believe if someone becomes selfish after getting power is because they already had less or no empathy to begin with. Those are the people who are yearning for power because they feel less important as a result of traumatic life and as soon as they get it they become pure evil. I personally I have seen people who got illusion  that they have power and they do start eating cookies differently! You can hear the arrogance in their munching! Not just eating but they way they walk changes too. Because they feel powerless and when they feel they have got power they try to act like powerful people but that acting is only of their concept of powerful people. The reality is far different from their concept, truth is when you are sincerely empathetic  the power makes you humble and modest.