The killer instinct is the ability within a person to take the life of another human being without the psychological inhibition normally associated with taking life. It is debated whether or not a killer instinct is inborn or whether it can be taught. It is argued that, while the mechanical action of killing can be taught, the actual instinct to do so remains inborn.

Often present in fighting, wars and confrontation, it is believed that some humans possess an ability to kill another human in order to achieve a goal. Examples include soldiers, fighters such as boxers, and mixed martial artists, who often display a form of this when trying to finish off an opponent, whereas it is argued that the main goal is to master self-defense in fighting arts. In Mixed martial arts, however, it is not the end goal to kill the opponent, so it is therefore not able to be included.