I will not return with thee: this was no lie, though he afterwards returned, because he spoke what he meant; his words and intentions agreed together, though afterwards he saw reason to change his intentions: compare Genesis 19:2,3: which may relieve many perplexed consciences, who think themselves obliged to do what they have said they would do, though they see just cause to change their minds.
Related Commentaries of 1 Samuel 15:26
1 Samuel 15:26
26 And Samuel said unto Saul, I will not return with thee: for thou hast rejected the word of the LORD, and the LORD hath rejected thee from being king over Israel.