Nothing much needs to be said about this one.
You only need to read the first three verses from 1 Samuel 15 to know for sure that the God of the Bible is evil. (Thank goodness he doesn't exist!)
Still, it is good idea to look at the story in context.
(Bible believers often tell us to look at the context. But context nearly always makes Bible stories worse, not better. This story is no exception.)
Samuel also said unto Saul ... Thus saith the LORD of hosts, I remember that which Amalek did to Israel, how he laid wait for him in the way, when he came up from Egypt. Now go and smite Amalek, and utterly destroy all that they have, and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass. 1 Samuel 15:1-3
Notice that Samuel speaks here for God. Why would anyone believe that then or now? Was it because it sounded like something God would say?
And if the genocide was so important to God, why didn't he tell Saul directly?
But let's assume that the Bible is right and Samuel quoted God correctly. If so, then (unlike many of God's killings) we know why God wanted to Saul to kill all the Amalekites -- because God remembered what their ancestors did hundreds of years before they were born (they defended themselves when the Israelites tried to kill them and take their land).
And Saul pretty much did as God commanded. He sent 210,000 soldiers to kill everyone and everything among the Amalekites: men, women, children, babies, animals -- except for the king, Agag, and some of the more valuable sheep, oxen, and lambs.
Saul gathered the people together ... two hundred thousand footmen, and ten thousand men of Judah. ... And Saul smote the Amalekites ... And he took Agag the king of the Amalekites alive, and utterly destroyed all the people with the edge of the sword. But Saul and the people spared Agag, and the best of the sheep, and of the oxen, and of the fatlings, and the lambs, and all that was good, and would not utterly destroy them: but every thing that was vile and refuse, that they destroyed utterly. 15:4-9
Saul killed the old men, the pregnant women, the children and babies. But he spared the king and some of the more valuable animals, and God would never forgive him for it.
Samuel cried his little eyes out all night and God repented of making Saul king.
Then came the word of the LORD unto Samuel, saying, It repenteth me that I have set up Saul to be king: for he is turned back from following me, and hath not performed my commandments. And it grieved Samuel; and he cried unto the LORD all night. 15:10-11
It's good to know that God repented. When you order someone to commit genocide for stuff that happened centuries ago, you should be a bit sorry about it.
But that wasn't what God was sorry about. God wanted all of the Amalekites to be killed. In some sick way it made God happy every time an old Amalekite woman or a little baby was killed.
No, God repented of making Saul king because he didn't kill everyone, and God wanted everyone killed.
And if you believe in the Bible, you wanted them killed, too.
[If this Bible story is true (which it isn't, of course), how many Amalekites were killed at God's command? I originally gave it the usual 1000 for a standard massacre, but Saul sent 210,000 soldiers to do the killing. He wouldn't send that many soldiers to kill only 1000 civilians would he? So I increased it to 10,000.]