1 Samuel 15:7-9 (NLT2) - Then Saul slaughtered the Amalekites from Havilah all the way to Shur, east of Egypt. He captured Agag, the Amalekite king, but completely destroyed everyone else. Saul and his men spared Agag’s life and kept the best of the sheep and goats, the cattle, the fat calves, and the lambs—everything, in fact, that appealed to them. They destroyed only what was worthless or of poor quality.
Saul and his men did not destroy all from the battle as God commanded. The law of setting aside everything for destruction was well known to the Israelites. Why ? We read this in Deuteronomy 20:16-18 this:"as the LORD your God has commanded, that they may not teach you to do according to all their abominable practices that they have done for their gods, and so you sin against the LORD your God.
Partial obedience , selective obedience is just another form of disobedience. The motivation behind this behavior was greed or covetousness. That seems like the motivation for a lot of sin these days.