Why would Capote present Herb Clutter first as a character, and then Perry Smith? What is significant about these two characters?

Capote's presentation illustrates the stark contrasts between the two men. First we meet Clutter, who is a well respected member of his community, a loving husband, a beloved father, and a successful business man. Later we meet Perry, who didn't even want to rob the Clutters, walk into the Clutter home and completely lose it because they had everything he ever wanted.... everything he didn't have.