1Samuel 18:8-9 And Saul was very wroth, and the 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.
It appeared to Saul that David was more popular in eyes of the people than Saul. The main cause for his continued animosity toward David was the song of the people, “Saul has slain his thousands, and David his ten thousands.” This popularity stirred up jealousy and rage in Saul, and he became afraid that David would take over the kingship. From that day he “eyed” David with suspicion.
In proud arrogance and fear he became unreasonably enraged against David, so much so that he tried to kill him, and then pursued him for months. Anger and jealousy opened the door for an evil spirit to come upon him. David had soothed Saul with his music, his son, Jonathan, loved David, and his daughter married David. Yet with all this working for him, Saul developed a demoniacal rage, simply because his pride was offended, his jealousy aroused, and his position threatened.
Sin increases many times over beginning from hurt feelings, often stemming from self-centeredness. Sin happens, but we would do well to admit sin early and, with the Spirit and the Gospel, stop the chain of destructive feelings and behavior.