God's Own Heart

1 Samuel 13:14
But now your kingdom will not endure; the LORD has sought out a man after his own heart and appointed him leader of his people, because you have not kept the LORD's command."

Here is where it fell apart for Saul, only one chapter after Samuel introduced him as king. It seems sad, yet it is inevitable. It's a matter of the heart. Saul must have possessed some kingly attributes, but his heart was wrong, and that's not something that can be concealed for long, if at all.
His downfall came on a matter of simple obedience. Samuel, whom he should have seen as mentor and spiritual advisor, gave him a test. The right answer was given: wait seven days for Samuel to come make the sacrifice.
Like Peter, Saul was sifted. He was presented with temptation almost too great to bear. He was surrounded by too many Philistines to count, and his army had deserted, hiding in the hills or outright running away. Those he had left were visibly scared. Samuel stretched his seven days to the last minute.
And Saul gave in, making the sacrifice he was not authorized to make.
He hadn't the heart.
The verse mentions God's heart. This is the first time David, not yet named, is called by this description. I just read of one of the kings three or four down the line, who is compared to David. David, it said, failed God in only one thing, the matter of Uriah the Hittite. Interesting that it always comes up that way, without mention of the woman.
I think it has so much to do with worship and purity of heart. David is never caught looking to other gods. He always turns to Yahweh, always wants to know his own God's heart.
Poor Saul was tossed by fear and pride.
He lost all in the sifting.