First of all I am sure there are many exceptions. There are many animals that don't kill period. Many of the grazing animals on the African savannah don't kill their own members and if there is aggressions, its generally males like wildebeest fighting over male supremacy to win the right to a female. Although I have seen films where some of the larger antelopes and wildebeests have killed by trampling hyenas and smaller carnivores trying to get one of their calves.
Okay so now the point I make is that in nature, aggression is part of the Darwinian code for survival of the fittest. In fact, aggression is very much a trait that is inherited via DNA and passed on to the next generation. What it does is it insures that the winner of a life and death struggle will live to pass on that specific aggressive trait to the next generation. There are many facets to it. In early humans for example, among hunter gathers, it could mean life or death of a clan, especially if food resources were scares. So killing the neighboring clan in cold blood murder meant more food for your clan and your progeny. We have an aggression trait that made homicide a favorable trait for early humans and well into modern times. When we developed agriculture and settled down, and civilization rose around agricultural basins... that murderous trait became a problem. But some of these psychologists are now recognizing is that it isn't some sort of mental defect... rather its some inherited behavior that lurks within us, and has always been with us and was at some point in our past beneficial to our survival.
Now as far as killing for the joy of it... well there are lots of exceptions there too. I have seen a video of a pod of Orca killer whales playing with a seal. They tossed it back and forth like a badminton shuttlecock. I have also seen videos of lion cubs toying with a small tiny antelope calf... literally biting it and tormenting that thing, but they had already eaten because their mother killed the mother of the antelope; which brings me to my favorite cat. My Himalayan cat I bought in 1982... a beautiful Blue point Himalayan long hair. Anyhow, she was the sweetest thing around humans, she would rub against your legs, demand to have her neck scratch, and every morning she would jump into my bed and sit on my chest and purr like a well tuned sports car until I got up and fed her. I never own a sweeter animal for a pet. But my God, she was the most wicked, mean, merciless huntress that would kill for the joy of it. I over fed that cat so she never needed to kill to eat, she was also finicky eater, but if she ever saw a small animal, a lizard, or a mouse, she would catch it and toy with it until it was dead. Never ate anything she caught. The first time was in Florida when she caught a series of lizards. We have these small brown anoles all over the place in Florida. She would catch one and gnaw off the legs and feet alive. Then other times she was really good at just playing with it. First time I realized how dexterous house cats were with their claws. She caught one and she would wear it out. She would allow it to escape, and then swipe at it and with one claw extended like an index finger, she nailed the lizard and dragged it back. She would do that over and over again tormenting that lizard. One time she caught a mouse, and played soccer with it. I though she was playing with a sock ball out of my laundry basket. But no it was a mouse that she tortured to death and then dropped it by my feet.