In the book “Is God a Moral Monster?" (this website too has some of its theories), the author talked about the exaggeration used in ancient Near Eastern writings as a way to explain why Joshua didn't commit genocide when entering Canaan. For instance, Joshua 10:40 is an example of such exaggeration.
So Joshua struck the whole land, the hill country and the Negeb and the lowland tand the slopes, and all their kings. He left none remaining, but devoted to destruction all that breathed, just as the Lord God of Israel commanded. (Joshua 10:40, ESV)
However, does that principle apply to 1 Samuel 15?
"Now go and strike Amalek and devote to destruction all that they have. Do not spare them, but kill both man and woman, child and infant, ox and sheep, camel and donkey.”... But Saul and the people spared Agag and the best of the sheep and of the oxen and of the fattened calves and the lambs, and all that was good, and would not utterly destroy them.... (1 Sam 15: 3,9, ESV)
Here King Saul is apparently punished for leaving survivors:
And Samuel said to him, “The Lord has torn the kingdom of Israel from you this day and has given it to a neighbor of yours, who is better than you.” (1 Sam 15:28a, ESV)
Though this time it is about lifestock, not human lives. However if the command to "leave none alive” (v.3) is interpreted as exaggeration, in view of typical Ancient Near East Warfare Rhetoric, isn't Saul not at fault for sparing some of the lifestock? (Apparently, Saul also spared a king, though Samuel [and God] seems to be more upset about the livestock)
Hence, if Saul was punished for leaving survivors, wouldn't that imply when God commanded "all to be killed," it literally meant so?