Augustine on Envy

Now these brothers, Cain and Abel, were not both animated by the same earthly desires, nor did the murderer envy the other because he feared that, by both ruling, his own dominion would be curtailed -- for Abel was not solicitous to rule in that city which his brother built -- he was moved by that diabolical, envious hatred with which the evil regard the good, for no other reason than because they are good while themselves are evil. For the possession of goodness is by no means diminished by being shared with a partner either permanent or temporarily assumed; on the contrary, the possession of goodness is increased in proportion to the concord and charity of each of those who share it. In short, he who is unwilling to share this possession cannot have it; and he who is most willing to admit others to a share of it will have the greatest abundance to himself. The quarrel, then, between Romulus and Remus shows how the earthly city is divided against itself; that which fell out between Cain and Abel illustrated the hatred that subsists between the two cities, that of God and that of men. City of God XV.5