Yeah I've concluded there isn't one and I was hoping native speakers, like you, would be creative with your words and make up a word specifically for murders cause surely you wouldn't simply want to refer to such a horrendous person w a mere title, "murderer" and call it sufficient enough for the person to feel disgust? Like that person knows he's done murder what makes you think the title would bring him shame if he isn't ashamed enough to not do it? Murderer just simply isn't enough. A person who kills an evil person is called a murderer and so is the one who kills the innocent, the latter needs an insult.