The Guardian says:
The story of Saul and the Amalekites is a case in point. It's not a pretty story, and it is often used by people who don't intend to do pretty things. In the book of 1 Samuel (15:3), God said to Saul:
"Now go, attack the Amalekites, and totally destroy all that belongs to them. Do not spare them; put to death men and women, children and infants, cattle and sheep, camels and donkeys."
Saul dutifully exterminated the women, the children, the babies and all of the men – but then he spared the king. He also saved some of the tastier looking calves and lambs. God was furious with him for his failure to finish the job.
The story of the Amalekites has been used to justify genocide throughout the ages. According to Pennsylvania State University Professor Philip Jenkins, a contributing editor for the American Conservative, the Puritans used this passage when they wanted to get rid of the Native American tribes.
This book says:
English conceptualization of Indian apostasy, and the figuring of Indians as Amalekites, was advanced through the depiction of Indians as remnants of the Ten Lost Tribes of Israel.
This book says:
Are there any historical records that say Christian colonizers viewed the American Indians as Amalekites?