OK, in response to Sher's request, I will post my theory. Now I haven't seen "Se7en" in ages, and I'm certain I'd be able to make a much stronger case if I'd just seen it yesterday, but I can remember most of it.
The fundamental tenet of the theory is that Kevin Spacey is not the killer at all, but just a psycho used as a pawn in the real killer's master scheme. So who is the real killer, you say?
Good question. It's Morgan Freeman.
Random list of reasons why:
- I started thinking it was him when he was making so many non-intuitive leaps of logic to track the killer.
- His reaction to finding the "killer"s headquarters was also odd. He didn't want Brad Pitt to go in there right away without reinforcements because he wasn't sure he hadn't left anything incriminating out there.
- Don't you think Spacey turning himself in was too easy? It was part of the plan. The ultimate final victim (and the final sin) was Brad Pitt himself, who Morgan Freeman thought represented the new breed of hotshot, self-absorbed cops.
- Freeman's bored and retiring. Time to put all his training to use for one final perfect crime.
- When the car arrives with the box, Freeman BUSTS ASS to be the first to get the box. Can't remember if Pitt actually looks in the box, but even if he does, it's just a glimpse. It's Freeman's reaction that tells him what he thinks he needs to know, and he's already in an agitated mental state from the buttons Spacey has pushed (according to the script laid out by Freeman).
Seriously, watch the movie again with this idea in mind and see how many things seem to back up what I'm saying. I developed the idea very early on, and I was 100% convinced by the end. I even wonder if there was an original script that revealed Freeman as the killer that got booted for being too depressing for American audiences.