Seven's creeptastic John Doe thinks he has been chosen by God to smite sinners and turn the world around. Um, no. This type of biblical-vengeance thing always turns out wrong—especially because the Bible states that it's not humanity's job to judge. (Didn't you read the whole Bible, JD?)
And our dynamic duo—Mills and Somerset—are on a very real mission to bring John Doe to justice. In fact, the whole last scene of Seven comes about because Mills and Somerset want to pin John Doe with the harshest sentence imaginable: Doe states that he'll plead insanity if the two cops don't follow him into the desert but says he will plead guilty if they do.
Questions About Justice & Judgment
Has Somerset given up on justice? Does he think justice is possible?
Is justice served by Mills shooting Doe at the end?
Is John Doe a vigilante? Does anyone care about the victims he murders? Would anyone ever see him as a hero?
Chew on This
Both Mills and Doe want to serve justice, but each man wants to do it in different ways.
By giving up on justice, Somerset has allowed injustice to happen.