He failed the test (1 Samuel 13)

“How foolish!” Samuel exclaimed. “You have not kept the command the Lord your God gave you. Had you kept it, the Lord would have established your kingdom over Israel forever.

Saul’s army became discouraged waiting for Samuel to come to Gilgal, and many of them left for home. Saul took matters into his own hands and offered the sacrifices to the Lord himself, despite Samuel’s clear instruction to wait for him. But a week had gone by, and Samuel had not come.

It was this “taking matters into his own hands” that proved to be his downfall.  The Lord delayed Samuel’s coming to Gilgal, so as to test Saul.  But he failed the test.  The Lord would now look for a new king —one who, as Samuel told him, was “after my own heart”.