Gregory the Great

For it is written of him: But by the envy of the Devil, death entered into the world (Wis. 2:24). Because he himself lost heaven, he envied the man who was created, and, ruined, he increased his own damnation by ruining others. The envious must be admonished to recognize the many falls to which the growing ruin subjects them, because, while they do not cast envy out of their hearts, they are led into open wickedness of deeds. For if Cain had not envied his brother's accepted offering, he would not have come to extinguish his life. Therefore it is written: And the Lord looked upon Abel and his gifts; but to Cain and his gifts He did not look. And Cain was very angry, and his face fell (Gen. 4:4).