That was the turning point, where the Lord rejected Saul and finally in intention passed the kingdom from him to David.
The self-principle goes too deep for the Lord to regard it lightly.
It is not just a matter of the person. It is there that the link with an entirely antagonistic spiritual system is found.
Amalek was such a link. Amalek had stood in the way of Israel when they came out of Egypt and were making for the land.
They had stood across their path in the attempt to frustrate the Lord's intentions of spiritual fulness for Israel, and that very people Amalek were the test case for Saul as to whether he was really wholly set upon the Lord or whether he had personal interests.
When, through Samuel, the Lord commanded Saul to destroy every vestige of Amalek, leaving nothing alive, Saul reserved the best of the herd and the flock.
He discriminated according to human judgment, to keep something that he fancied, that he thought was good.
He set his own judgment over against the judgment of the Lord because of this self-principle that was in him, thus proving that in principle he was one with Amalek, that is, he was not set upon all that the Lord was after.
The Lord was seeking to bring Israel into the land, that is, to spiritual fulness.
Amalek said 'No'. Saul and Amalek found themselves one in principle. He spared them.
But see what Samuel does to Agag, king of the Amalekites! - he hews him in pieces before the Lord. There is NO COMPROMISE there.
~T. Austin Sparks~