The Evil, Envious Eye

As they were coming home, when David returned from striking down the Philistine, the women came out of all the cities of Israel, singing and dancing, to meet King Saul, with tambourines, with songs of joy, and with musical instruments. And the women sang to one another as they celebrated,
“Saul has struck down his thousands,
and David his ten thousands.”
And Saul was very angry, and this saying displeased him. He said, “They have ascribed to David ten thousands, and to me they have ascribed thousands, and what more can he have but the kingdom?” And Saul eyed David from that day on.
The next day a harmful spirit from God rushed upon Saul, and he raved within his house while David was playing the lyre, as he did day by day. Saul had his spear in his hand. And Saul hurled the spear, for he thought, “I will pin David to the wall.” But David evaded him twice.
Saul was afraid of David because the LORD was with him but had departed from Saul. So Saul removed him from his presence and made him a commander of a thousand. And he went out and came in before the people. And David had success in all his undertakings, for the LORD was with him. And when Saul saw that he had great success, he stood in fearful awe of him.
(1 Samuel 18:6-15 ESV)
Persecution of God's people is a thread throughout history since the Fall.  Here, David is blessed by God for his willingness to freely and courageously serve God and His people.  When others saw that God was blessing him, they took notice.  For most there was no competition; they were just glad to have David on their team.  But for Saul it was a cause for envy, fear, and hatred.  He wanted David out of his sight and out of his life - permanently.  That attitude leads to persecution - mistreatment for doing good and seeking to be right before God.  Sometimes it comes from those who are part of the same team, who at least proclaim to belong to God's people.  Then it is a double blow: it hurts sincere followers and it destroys the testimony of God's people.  Persecution: don't be surprised when it comes, and don't allow yourself to become its tool.