The funny underlying message
So basically this guy figured out how to get away with murder by simply evading the police's methods to prove against him.
Nothing too elaborate. He just used what everyone knows to be what police uses as evidence, and manipulated that against them.
When Robert is trying to solve the case based on common sense arguments, one of the cops says it himself: that's not what they need, they need DNA and fingerprints, and handwriting (lol). That's how you prove anything. That's a big ass weakness if someone figures out how to evade that.
For me this movie was somehow a critique to the over objective, standardized and braindead guidelines based on which most of society works. Rules rule the world. I am not saying we should arrest someone based on circumstantial stuff, but behaving like machines certainly limits our full potential. I guess that's why mostly everyone involved in the case got so frustrated.
Great film :)