In essence, an autobiography is a chronological telling of one's experience, which should include phases such as childhood, adolescence, adulthood, etc., while a memoir provides a much more specific timeline and a much more intimate relationship to the writer's own memories, feelings, and emotions.
He was angry with the way Indians were treated by the Spanish and urged the Spanish authorities to show more humanity to them.
Álvar Núñez Cabeza de Vaca was a Spanish explorer and the first European to step foot into (what is now known as) Texas and from the day he arrived, he and his men faced continuous attacks by the Indians that were already there. Eventually, his men were overwhelmed and he was one of the four survivors of the expedition.
In 1536, the survivors were found and rescued by a Spanish slaving party, and when he returned to Spain for the first time, he was outraged by the treatment of Indians and asked that they be treated better.