Why did God repent of making Saul king?
Because he failed to kill all of the Amalekites and their animals.
It repenteth me that I have set up Saul to be king: for he is turned back from following me,
and hath not performed my commandments.
1 Samuel 15:11the LORD sent thee on a journey, and said, Go and utterly destroy the sinners the Amalekites, and fight against them until they be consumed.
1 Samuel 15:18Thus saith the LORD of hosts ... go and smite Amalek, and utterly destroy all that they have,
and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass.
1 Samuel 15:2-3
It repenteth me that I have set up Saul to be king: for he is turned back from following me,
and hath not performed my commandments. 1 Samuel 15:11
the LORD sent thee on a journey, and said, Go and utterly destroy the sinners the Amalekites, and fight against them until they be consumed.
1 Samuel 15:18