A lot of people think that Gary Ridgway killed people because, well, he liked to kill people. Other people believe that he killed prostitutes because, at least partially, he wanted free sex. Look at it from whatever perspective you want, but from my angle, I'll go with the latter.
Early on in the film, Willem Dafoe, who seems to be a college-professor-slash-murder-detective, warns his students with something like the following: "When you view a murder scene, take your time and look at it for awhile before deciding why the killer killed. After all, he may not even know himself." Indeed. When Dave Reichert asked Mr. Ridgway if he was different than other people, if he was perhaps missing something vital in his personality that caused him to be a serial killer, Mr. Ridgway just shrugged his shoulders and more asked than answered, "Maybe that whole 'caring' thing?" (I found that line darkly humorous myself, as probably did the filmmakers of "The Hunt for the Green River Killer." I don't believe Mr. Reichert found it amusing, however.)
And this is the setup for a film that I found quite underrated. The theme would probably be simply "perspective." Mr. Dafoe, who I think has chosen a few roles lately that have gone off the rails -- see "Antichrist" and "444: Last Day on Earth" to name but two -- chose to play another character here that is slightly off, but not completely. He's much more controlled, and even though his character has had a dark and troubled past, plays it close to the vest. At first, I thought that he was one of those "gatekeeper" guys, or someone that purposefully hides information at work to stay ahead of his peers. But that would be too simplistic, kind of like those people above that think that Mr. Ridgway killed because, well, he liked to kill people. Like Mr. Ridgway, Mr. Dafoe's character is certainly more complex than that.
This film is surely "artsy," and usually, that could easily be a turn off for me. But while this film is surely not completely realistic -- I don't believe that any serial killer could ever completely pose a scene with the foresight that this one does -- I don't think that this film has to be completely realistic. Just like all of the "artwork" that the serial killer here "creates," this film could be argued to be "art" as well. At least I saw it that way.
I took an art appreciation class in college, which probably helped a little in understanding some of the complexities and terminology in this film. While I am far from an expert, I'll bet art students would probably appreciate this film the most. But even from my limited perspective, I found this film quite interesting, and I'll give it a pretty big recommendation.