John Gill Commentary 1 Samuel 15:19

"Wherefore then didst thou not obey the voice of Jehovah, but didst fly upon the spoil, and didst that which was evil in the sight of Jehovah?" — 1 Samuel 15:19 (ASV)

Why then did you not obey the voice of the Lord
Who had made him king, and sent him on this errand, and gave him such plain directions, and such strong orders to make an entire consumption of Amalek:

but did fly upon the spoil ;
like a bird of prey, such as an eagle or vulture, not to devote it to the Lord, by an entire destruction of it, but to seize it for his own use, as being greedily desirous and covetous of it:

and did evil in the sight of the Lord ?
by disobeying his commands, from whose sight nothing can be hid.