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This is the very FIRST MoviePooper entry!
The serial killer is is an uncredited Kevin Spacey, who turns himself in after the fifth victim is found, and offers to take the detectives on a "field trip" to demonstrate how his "Seven Deadly Sins" spree will conclude. Out in the desert, he starts to ramble about how much Envy he has for the life that detective Mills (Brad Pitt) shares with his wife Tracy (Gwyneth Paltrow), reveals that he knows about Tracy's pregnancy (a fact that Mills himself didn't know), and then gives Somerset (Morgan Freeman) a box with Tracy's head inside. Mills, representing Wrath, shoots and kills Spacey, which is just how he planned the killings to end.
Thanks, Andy I!
For all of your guidance and kind words, Pooper #1 is dedicated to you!
Read more about this film at The Internet Movie Database.