The difference between good and evil individuals is simply the difference between singular and plural.
They're flip sides of the same coin: the goal of each is advantage for the individual. A "good" person attempts to attain that advantage by working towards the benefit of his society, which benefit will also apply to him. An "evil" person, on the other hand, works directly for his own good.
Well, it seemed more profound at three in the morning; I guess you had to be there.