I've always believed the most dangerous serial killers are never known and most likely a very young female genius or a very old lady. Whether people want to be honest or not, there is a general bias that most likely the serial killer is a single male of the highest population demographic in their area, but many guys can get away using silent methods due to being more physically strong (this is why many of them prey on hookers throughout history, someone physically weaker that society doesn't care about) while a woman would have to be more subtle because she has has to have a smaller margin of mistake. In a sense an assassin is an emotionally detached killer who doesn't get off on killing their victims, but in the end they both are same thing, predator of people.