What makes a killer?
Any living soul can become one. Take away everything they love, give them no reason to live, make them desperate, and they can be taught to kill.
But the natural born killer will have the desire, and a compelling need to kill, manifesting itself throughout childhood as a latent yearning, not fully understood. Only on reaching maturity will this desire require satisfying.
And how can a natural born killer stand out in a species that has made murder into an art form?