Probably because he needed evidence? If he went without one, the police might not take him serious because the psycho is a famous figure in society I think. And if they did believe him and they went to the guy's house to inspect, they may not find any clue cos this guy is hella smart in covering his tracks. The serial killer would have also known that the mc went to the police to report him which would spell trouble for the mc
As for me, from the first of the job, I would have changed my apperance and skipped the damn country altogether if I had the chance. That's some crazy motherf**ker right there but that's just me ( ̄∇ ̄")
i get it, we need him to be stupid to advance the plot but why the hell would he not report the murder???