John Doe is the killer, played by Kevin Spacey. He willingly turns himself in at the police station. He tells the police he will take them to the remaining two bodies if Pitt and Freeman's characters go with him. They drive out into the middle of nowhere, and John Doe's plan comes to fruition: The sixth sin is Envy, John Doe's envy of David Mills (Brad Pitt) life. So he (John Doe) killed Gwyneth Paltrow (Pitt's wife), put her head in a box and sent it to Brad Pitt.(We don't actually see what is in the box, but it is most likely her head, since that's what John Doe tells ) The seventh sin therefore is Wrath; Brad Pitt finds out what Doe did and in his wrath shoots him. Morgan Freeman's character, who was due to retire, decides to stay with the police after all.