"A genius thriller that reflects the corrupt morality of society perfectly." - My interpretation of the movie:

Rating: 9/10 (2 April 2020)

If you are looking for a story-based movie, this will not be for you. If you are looking for something with a deeper message and lots of metaphors and reflections of wrong judgement, you will fall in love with this masterpiece. Here is my interpretation:
Joe, being drawn as the 'psycho serial killer' initially, is murdering his victims in accordance with the seven deadly sins. After his first 5 crimes in which he 'punishes' the victims, he goes for detective Mills' (Brad Pitt) wife. That is his sixth crime. When Mills finds out about it, he kills Joe in his own violent way (in fact, he shoots a whole magazine of bullets), and Joe's series of crimes complete.
Here, the message is that when someone that is judging others because of their 'crazy, psychopathic behavior', they are underrating or not understanding how 'personal' that person has taken the sins that the victims have committed. If Joe had killed some other random, Mills would have never shot him. He kills him because it is about his wife. However, that is why Joe has been killing in the first place: he takes the sins of all those victims very personal; unlike Mills who is lacking the empathy that Joe has and only decides to kill when the victim is his own wife. Here, Joe proves that Mills is basically no different: he also kills when it comes to a sin that is bothering him so much that he cannot live if he does not do something about it. That is what society is about: we judge people easily on behalf of their acts, but most of the time, we are no different.
Interestingly, Joe's sixth victim, Mills' wife, is also not a pure good person. She was considering having an abortion without telling his husband Mills, who is the father of the baby, anything about it - which is also a sin. You can clearly see the smile on Joe's face when he learns about the fact that Mills did not know about the pregnancy.
I think that Somerset (Morgan Freeman) could be a metaphor to the 'good and right part of Mills'. I think the scene where Somerset sits on Mills' desk is really key to make me think so. Most of the time, Mills wants to ignore that good part inside him, but when Mills' wife calls him, she wants to speak to Somerset (the good side in Mills), invites him to dinner, and Mills' wife opens herself about the pregnancy to again Somerset, the good side in Mills. From time to time, Somerset has tried to convince Mills not to do the wrong thing (first at Joe's apartment he told him not to break the door, and in the end not to kill Joe), but ultimately he failed.
In short, I think this is an absolute masterpiece and I am sure there are still a lot of things that I have not noticed about the movie. The only reason I don't rate it as a 10 is because of the acting that was not the best knowing Freeman and Pitt's potential. Maybe it's because they did not have enough time in their scenes to 'take us in'. Overall, David Fincher is a genius.