What was the first major step in Saul's downfall?

The first major step in Saul's downfall was his disobedience to God's commands, particularly when he spared King Agag of the Amalekites and kept the best of their livestock instead of destroying them as instructed. This act of defiance led the prophet Samuel to declare that God had rejected Saul as king. This pivotal moment marked the beginning of Saul's decline, as he struggled with jealousy and paranoia, especially towards David, who was anointed as his successor.