No one knows for sure since no reason is given in the text (the
T'anach/the Bible). Therefore the question is open to conjecture
within the commentaries and midrashim. @msh210 gives an answer
from the commentaries of Rabbi Moshe Alshich. Another answer: it
was "professional courtesy" -- while Saul was willing to kill
the people, he decided to spare his kingly equivalent, Agag. He
also decided to spare the animals for later use as sacrifices.
(But spoils were not allowed from this battle.) Both decisions
were examples of Saul's lack of commitment to follow Gd's
specific requests. Another example: (From wikipedia) According
to 1 Samuel 10:8, Samuel had told Saul to wait for seven days
after which they would meet; Samuel giving Saul further
instructions. But as Samuel did not arrive after 7 days (1
Samuel 13:8) and with the Israelites growing restless, Saul
started preparing for battle by offering sacrifices. Samuel
arrived just as Saul finished offering his sacrifices and
reprimanded Saul for not obeying his in