Why did God order to complete annihilation of the Amalekites in 1 Sam 15? Why order the Israelite army under Saul to "blot out their memory from under heaven?" Imagine you're an Israelite solider. If you're going to do your job correctly and obey the command of the Lord you and your platoon needs to raid a village, burn it to the ground. After breaking the door down, you first need to kill the man of the house. After him, take out the wife (maybe she's pregnant?), then the baby in the crib, the bed-ridden grandfather, the servant, and don't forget Mr. Whiskers the Cat. When you're finished in the house, your job is only half done. You need to go around back and care of the animals, but on your way, you find an Amalekite boy playing with his trucks - take him out. Then proceed to slaughter the cows, donkeys, camel and chickens.
Once you've made sure that nothing left is breathing. Move onto the next house.
In our modern language this type of act is called by many words: ethnic cleansing, genocide, holocaust. Take your pick. Such people who make this type of war are considered sociopaths, utterly depraved and terrible men against whom the world rushes to hang from the gallows.
But this is our God. How will we vindicate him? In our post WWII and 9/11 days, the thought of holy war, terror and ethnic cleansing freaks us out. The God of Abraham is not the most in-vogue figure around, and things are only going to get worse.
The reason given in the Bible for this act against the Amalekites was relatively quite simple: They were godless men who attacked the Israelites as they were coming out of Egypt, worn out and weary (Exo 17, Deut 25). God remembered that assault and made sure that Israel remembered it too. He vowed to pay them back for their offence once they settled in the land as a nation.
It was a 500 year grudge. Our God bears grudges. This is negative and repulsive to our 21st century sensitivities, and we want so much to ignore this aspect of God. To some Christians, this has led to embarrassment and a reformulation of theology altogether - one that keeps God "innocent" of such behaviour.
But this is our God. And before we slam down the gavel to judge him, let's consider it a bit more. The wrath of God is a function of two things: God's holiness and his Love. In his terror against the Amalekites, he shows us his disgust for godless sinners. In his 500 year-long grudge, he shows us his love toward his people. God is certainly good, and loving, and forgiving ... to Israel. and to us. Israel receives love. The Amalekites received wrath.
Is this only the God of the Old Testament? Not so! Consider another place where the wrath and love of God was expressed at the same time: The Cross. The ultimate display of God's wrath and love. Jesus received wrath. We received love. How terrible is the grudge of God, and how precious is the One who bore it for us.