The Serial Killer has the ability to kill one player each night. The Serial Killer must eliminate the Town and other hostile factions in order to win.
Try to even out the balance of power between opposing factions, such as the Town, Mafia, Triad, Cult and etc., keeping them from gaining too much strength.
A target being protected by a Doctor can still be killed if 2 attacks hit the target in the same night, targeting players you think other evil roles will try to kill should practically guarantee their deaths.
Pay special attention to the Roles List. If there is no confirmed Serial Killer or Neutral Killing, it may be beneficial to stay hidden until it's late into the game. This idea can also work if there is a confirmed Serial Killer and there are several Neutral Evil or Neutral Killing roles placed into the game. If there is a second Serial Killer, it is important to try and figure out who it is, since Serial Killers win with each other; no matter how many there are.
Try to guess who other evil players will try to kill at night, and attack someone else. It is important to maximize the number of deaths in a night.
Keep in mind that you have to balance the Town/Evil ratio.