When I was young they had found a solid correlation between murderers and people who would torture other animals for pleasure. Of course they distinguished between the killing of humans and animals, but since we are animals, if a person can kill an animal without any guilt or feeling of remorse, they can kill a human animal as well. The fault does not lie in admitting we are animals, it lies in the fact that so many humans still ignore the fact that we are animals.
All life must kill other life to survive, that is just the way life works, however no life has shown it truly enjoys the act, except for humans. Often people will attempt to say that other animals will kill for no reason, this is patently false. Typically when an animal kills there is some instinctual drive that triggers this act, usually that trigger is hunger. The sad fact is that even killing vegetables should require at least a second thought, this is a living organism that was sacrificed so we could eat. In the modern era we have the luxury of separating ourselves from these disturbing facts a bit; meat is already slaughtered and cut; veggies are already dead or dying when we purchase them.
This is not inherently a bad thing, however it has produced a very disturbing trend for the more affluent humans in the world; trophy hunting, or killing for fun. Ask any real hunter and they will tell you that they consume the meat, the trophies they collect are parts they can't eat, they don't hunt just for the trophies. The only people who could, and would, hunt solely for the trophies are killers, not hunters, and these are killers capable of going that one very tiny step to homicide.
It is a very tiny step too, once they remember that humans are animals, they will blur this boundary we have created between us and them; they will see humans as the ultimate trophies. The sad fact is that the killing is not even the worst ones, there are those who will abuse helpless animals for amusement as well; beating, raping, tormenting them for long periods of time, and enjoying it. These people could be anyone, but they will eventually abuse humans as well, human children most likely as they are incapable of facing something that could fight back.
This is why we must stop animal abuse, real abuse. Using animals for testing is not entirely unethical, and we do need to kill other animals for food. However, killing or abusing other animals for pleasure is truly a dangerous mind. In the past I had said that there are times and places such minds could be beneficial, however, doing this for pleasure is not one of those times. They could work in a slaughter house, no one says you can't enjoy your job. They could join the military, they even supply the guns and ammunition.
Recently I had learned that "caged hunting" is practiced in the world. That is the worst form of savagery a human can be capable of. Not only are they only killing for pleasure, the animals are not even given a fair chance to evade them, or fight back. A true hunter would dive in, with nothing but a knife, and wrestle a wild animal, winner takes all style. These trophy hunters are simply spineless psychopaths, a danger not only to other animals, but to us as well.