Not even this act of benevolence moved Saul, however. Again he gathered an army and again he set out in pursuit, until he was a second time apprehended by David in the cave where he more clearly revealed his own iniquity and made the virtue of David even more resplendent. Envy is the most savage form of hatred. Favors render those who are hostile to us for any other reason more tractable, but kind treatment shown to an envious and spiteful person only aggravates his dislike. The greater the favors he receives, the more displeased and vexed and ill-disposed he becomes.
Source: CONCERNING ENVY