The antagonist is George Harvey, the neighbor who rapes and kills Susie.
An antagonist is a villain, or a bad guy. The antagonist presents a conflict for the protagonist. In this case, the protagonist is fourteen year old Susie, who is killed by her neighbor George Harvey.
My murderer was a man from our neighborhood. My mother liked his border flowers, and my father talked to him once about fertilizer. My murderer believed in old-fashioned things like eggshells and coffee grounds… (ch 1, p. 6)
Mr. Harvey was a loner and somewhat strange. His antisocial behavior was not as obvious as with some, but he kept to himself making dollhouses. He raped and murdered Susie when she tried to cut through a cornfield. He dismembered her and left her body there to rot. She looked down on him and her family members from Heaven, keeping track of the investigation.
Although Susie only has a physical conflict with Harvey in the beginning of the book, he is the reason she is dead and therefore continues to be the antagonist as her friends and family, and the police, try to find out what happened to her and bring her killer to justice. Harvey is eventually killed by a falling icicle, which Susie considered the perfect way to murder someone because the weapon melts.