Why does a 1st level commoner have all 18?

Well there are several classoc "genres" of serial killers:
There is your classic random loony or crazy man.
There is the fanatic, killing off people for some obscure "crime" that they did, real or imagined (you might want to have your madman be a fanatical, nature-worshipping, modern-man-hating druid wannabe considering his ranks in the knowledge (nature) skill. He was so fantical in his devotion to nature and so mindless in his enforcement of Nature's Will even the Shadow Druids would not take him. So he fights Nature's fight alone without any help from the "Pretenders".
There is you crazed, misguided cultist bent on getting blood sacrifice to some Dark God or Overlord. Perhaps that is what granted his mind-bogginly high ability scores.
There is your paranoid schizophrenic, thinking everyone is out to get him; the guy who looked at him funny was certainly an assassin hired by the Ominous Lizard People.
Then there is the serial-killer wannabe, whose motif is just a cheap rip-off of some great, *real* serial killer, and this guy just does it because it makes him get a sense of power he cannot get in his everyday pathetic life.
Or what might be cool is to figure out a combination of the above. Perhaps he is a paranoid, fanatical slave to some dark nature goddess (real or imagined or maybe even a demon lord just playing around with the guy) trying to imitate some long-dead serial killer he believes to be serving nature. Serial killers are known for their calling cards- maybe some ritual scars that mimic a local druid groves tatoos or magical signs; this helps the plot in that the killer unknowingly places the blame on the local druids. Of course the druids don't want to talk because they threw this loony out and it is an embarrasment. Or maybe leaving behind some sort of herb.