John Gill Commentary 1 Samuel 15:26

"And Samuel said unto Saul, I will not return with thee; for thou hast rejected the word of Jehovah, and Jehovah hath rejected thee from being king over Israel." — 1 Samuel 15:26 (ASV)

And Samuel said to Saul, I will not return with you
Not being satisfied with his repentance and confession, he still extenuating his sin, and laying the blame of it on the people. This he said by way of resentment, and as expressing his indignation at him, though he afterwards did return with him on a change of his mind; which a good man may be allowed to make, without any imputation of falsehood or a lie to him:

for you have rejected the word of the Lord, and the Lord has rejected
you from being king over Israel ;
which is repeated from (1 Samuel 15:23) for the confirmation of it, and to let Saul know that his pretended confession and repentance had made no alteration in the decree and sentence of God respecting the kingdom.