Andrew Kevin Walker is not a city person. He grew up in Pennsylvania, and after graduating from Penn State, he moved to New York City and worked at Tower Records. New York City in the mid-'80s + working retail = deep, burning hatred for all of humanity.
Walker translated this hatred into his script for Seven, a film that's not as much of a serial killer movie as one simply about humanity's capacity for evil—something anyone who works retail long enough can understand.
In addition to Seven, Walker wrote other dark scripts for the horror anthology show Tales From the Crypt and the films 8MM (1999) and Tim Burton's Sleepy Hollow (1999), as well as rewrites on Fincher's Fight Club (1999) and the space horror movie Event Horizon (1997).
However, some people just can't handle Walker's level of depravity. He encountered resistance from New Line Cinema regarding the ending of Seven, and only when Brad Pitt intervened did they agree to keep Gwyneth's head in the box. But when 8MM director Joel Schumacher (the guy who put nipples on the Batsuit in the 1997 flick Batman & Robin) made changes to Walker's script, Walker walked and barely wrote again for over a decade. (Source)