This is just the stupidest thing ever.
Town doesn't even know who to lynch, let alone who to let a Serial Killer go out and kill! Instead of losing one Town player each Day/Night Cycle, we'll be losing two + the mafia shots. Obviously River will follow through with whoever we tell her, because what does it matter to her if she hits Town or Mafia, she has to shoot them all anyway. Thus, our numbers will drop significantly and essentially River can get around this and win.
Without the Serial Killer, Town has had no problem winning games. Lynch River and get it over with.