In the case of Cain and Abel, there was no rivalry in any cupidity for the things of earth, nor was there any envy or temptation to murder arising from a fear of losing the sovereignty in both were ruling together. In this case, Abel had no ambition for domination in the city that his brother was building. The root of the trouble was that diabolical envy which moves evil men to hate those who are good for no other reason than that they are good. Unlike material possessions, goodness is not diminished when it is shared, either momentarily or permanently, with others, but expands and, in fact, the more heartily each of the lovers of goodness enjoys the possession the more does goodness grow. What is more, goodness is not merely a possession that no one can maintain who is unwilling to share it, but it is one that increases the more its possessor loves to share it. (St. Augustine, City of God, XV, 5)
This is related to what I call the “fornicators’ dilemma,” which I commented on some time back.