Carnivores kill to live, and the instincts that allow true carnivores to survive include the killer instinct. This is more than lack of empathy for the prey, it is joy in the hunt. The thrashing agony of a dying gazelle in the mouth of a lion touches not the heart of the lion, who is focused on his hunger and what is known as the thrill of the hunt. Without this thrill, carnivores would be less likely to survive, as the fatigue that comes from relentless hunting trips, most often unsuccessful, would overcome hunger. Thus, for carnivores, the killer instinct is inborn, natural, and therefore more easily forgiven. Carnivores that hunt to live are unlikely to kill strictly for sport, as when satiated they lie about sunning themselves. However, the killer instinct adds excitement to the long and frustrating hunts, so once up on their feet and hunting, carnivores are motivated to keep going until they come home with the catch.
The killer instinct has often been ascribed to humans, to explain human behavior that most find shocking. Beyond sadism, some humans kill just to run up the numbers, casually, for trifling reasons, and without a backward glance or twinge of remorse. This is romanticized into something called the killer instinct, akin to the noble lion or savage stalking tiger. How else to explain a human who casually kills others? This is akin to suicide, developing a casual attitude toward life because there is a desire for death. Many who want to die lack the courage to enter into the final fray, not so much due to anticipation of the final agony, but fear of living through the attempt, maimed and out of control, unable to finish the job. Casual killers are simply hoping that someone will return the favor, and eventually, someone does.