Charles Spurgeon Commentary 1 Samuel 18:7-9

"And the women sang one to another as they played, and said, Saul hath slain his thousands, And David his ten thousands. And Saul was very wroth, and this saying displeased him; and he said, They have ascribed unto David ten thousands, and to me they have ascribed but thousands: and what can he have more but the kingdom? And Saul eyed David from that day and forward." — 1 Samuel 18:7-9 (ASV)

Saul has slain his thousands, and David his ten thousands.

And Saul was very angry, and the saying displeased him; and he said, They have ascribed to David ten thousands, and to me they have ascribed but thousands: and what can he have more but the kingdom? And Saul eyed David from that day onward.

This shows how envy will destroy a person's own peace of mind, as well as make them plot against the one they envy.

Instead of being thankful to God for sending him such a valiant champion as David to deliver both himself and his people, Saul is full of malice towards the young hero simply because he receives his due praise for his victory over the giant. "Saul eyed David" with an evil and envious eye; he looked askance at him, and determined to do him harm whenever he could.