Saul died because he was unfaithful to the Lord

1 Chronicles 10:1-14

But his armor-bearer was terrified and would not do it; so Saul took his own sword and fell on it. When the armor-bearer saw that Saul was dead, he too fell on his sword and died. So Saul and his three sons died, and all his house died together.

When all the inhabitants of Jabesh Gilead heard what the Philistines had done to Saul, all their valiant men went and took the bodies of Saul and his sons and brought them to Jabesh. Then they buried their bones under the great tree in Jabesh, and they fasted seven days.

Saul died because he was unfaithful to the Lord; he did not keep the word of the Lord and even consulted a medium for guidance, and did not inquire of the Lord. So the Lord put him to death and turned the kingdom over to David son of Jesse.

Why?

Because Saul was unfaithful to the Lord, the Lord put him to death and turned the kingdom to David son of Jesse.

How?

To be a faithful man to the Lord, Saul focused on himself and did not care about the word of the Lord. Even before the war, he consulted a medium for guidance. I should consult the Lord for everything. For I know the Lord loves me.