He was chosen by God to be king. Saul then chose to be unfaithful. Not a good combination. Hmmm, life or death? I choose life.

1 Chronicles 10:13-14

So Saul died for his unfaithfulness which he had committed against the Lord, because he did not keep the word of the Lord, and also because he consulted a medium for guidance.
But he did not inquire of the Lord; therefore He killed him, and turned the kingdom over to David the son of Jesse.

Bible Gateway link