He [David Fincher], along with Mike De Luca and Brad Pitt and Morgan and Kevin Spacey, fought-fought-fought to save that ending, and they are the reason that that ending was maintained and is the ending that’s in the movie. Without them, no one was going to listen to me, the lowly screenwriter — that’s just the way it goes.
Because who would know better whether or not an ending works well thematically with everything that came before it then the guy who has spent the past couple of years going over it with a fine tooth comb.
The ending of Se7en had to be what it was because everything prior led up to that moment. Pity's character was a hot head and it was Morgan's character who constantly kept him in check. It was only once Morgan finally had his moment of impulsiveness (the slapping of John Doe) that Pitt's character was finally allowed off his chain. Those moments meant something. As despicable as Doe's actions were, they weren't a threat as long as more level-headed men were around.
Once that departed, so did any sense of humanity.
Se7en should always remains shining example of why it is important to have a purposeful complete narrative first. And why it is important to fight every step of the way for that story.