1 Samuel 13:5-14
Bitterness is never a proper response for a child of God. It poisons the mind, emotions, and spirit. When did this resentful attitude take root in Saul? Let's look at two events in his life for clues.
Saul couldn't go to battle until Samuel arrived to offer the burnt sacrifice as God had commanded. Before the prophet arrived, though, Saul's men became scared of the enemy and started to leave, so the king took action and made the sacred offering himself. He may have thought, This has to be done right away. Why shouldn't I do it? He failed to see the necessity of obeying divine commands to the letter. God saw this as a serious act of rebellion, which brought judgment: Saul's kingdom wouldn't endure. God would one day appoint someone else to take over as ruler instead of Saul's descendants. Imagine Saul's reaction to hearing the kingdom would not belong to his family.
During the war against the Amalekites, King Saul once again failed to follow the Lord's instructions fully. When Samuel confronted him about his misdeed, he lied and claimed that he had obeyed. Later, however, Saul attempted to justify his disobedience. His unrepentant heart grieved the Lord and resulted in further judgment: he, the king, was now rejected by God (1 Sam. 15:10-11, 26).
I believe Saul's descent into bitterness began with the news that his disobedience would cost him the kingdom. Be sure to take your disappointments and anger immediately to the Lord. Pour them out before Him, and ask that He help you let them go. Your spiritual health depends on it.