The tenth commandment requires that envy be banished from the human heart. When the prophet Nathan wanted to spur King David to repentance, he told him the story about the poor man who had only one ewe lamb that he treated like his own daughter and the rich man who, despite the great number of his flocks, envied the poor man and ended by stealing his lamb. Envy can lead to the worst crimes. "Through the devil's envy death entered the world":
We fight one another, and envy arms us against one another.... If everyone strives to unsettle the Body of Christ, where shall we end up? We are engaged in making Christ's Body a corpse.... We declare ourselves members of one and the same organism, yet we devour one another like beasts.
Source: Catechism of the Catholic Church (Vatican.va)