He remembers

Then he said to the Kenites, "Go away, leave the Amalekites so that I do not destroy you along with them; for you showed kindness to all the Israelites when they came up out of Egypt." So the Kenites moved away from the Amalekites. 1 Sam 15:6

Notice that God never forgets when you have obeyed.

Some background

The Kenites were a Midianite tribe of blacksmiths, resembling "the gipsies of the Middle Ages".

In Exodus, Chapter 18, when the Israelites were coming out of the wilderness, Jethro, the priest of Midian and the father-in-law of Moses, met Moses, blessed him and thanked the Lord for what the Lord had done for the Israelites. In Numbers, Chapter 10, Moses asked Hobab, his Midianite brother-in-law, to lead the Israelites through the wilderness. These Kenites were nomads and knew that territory like the palm of their hand. They joined the Israelites and led them through the wilderness. Finally they settled with the tribe of Judah in the Promised Land. Although by this time they had been dwelling with the Amalekites, 400 years before they had made a choice. That choice was to follow God and be God's instrument.
God remembered that. Now, 400 years later, the Kenites are separated and spared during the slaughter of the Amalekites.

Notice that God never forgets when you have obeyed.

Notice that in the protection of the Kenites and the destruction of the Amalekites not only the fulfillment of a very specific promise of God, but also the fulfilling of God's covenant with Abraham (Genesis 12:1-3). In this covenant with Abraham, He promises to bless those who bless
Abraham and his descendants (Kenites), and to curse those who curse Abraham and his seed (Amalekites).

Notice that God never forgets
His promises!