Seven is all about power dynamics. Who is in charge? John Doe and his crazy killings? Detective Mills? Detective Somerset? The answer is: none of the above. David Fincher is in charge here, from beginning to end. He likes the idea of omniscience, making it seem like he's playing god, in a sense.
However, he shoots the movie to make it look like John Doe is in charge. If anything, the detectives are out of their element from the beginning, and even the elements are against them. In Seven, it's raining almost the entire time, creating a dark atmosphere that never lets up—as if brutal serial murders weren't dark enough.
Fincher mostly shoots with Steadicam, but Seven features the most handheld camera shots of any of Fincher's films, mostly used during chase scenes. But as Tony Zhou points out, the detectives are shot in handheld, creating a chaotic, out-of-control feeling, while Doe is shot on a tripod, cool and calm.