The animalia kingdom differs from the other members of our domain - primarily the plantae, fungi, and amoebae – in that we possess the active capacity to consciously kill. There are five types of animals who will kill, I have concluded. Each one I have classified in accordance to their rarity and characteristics or motivations for executing this act. When I say this, I mean to say that the five classifications are a direct result of mental state; no one species is exempt from falling into any of the five classes. Nor is any one individual set into one group for life. Indeed, I myself have belonged to each of the classifications at one point or another in my long life.
The first class of animal, by far the most common and "natural" of the taxonomy, will kill as a means of survival. Be it for food, self-defense, or as an indirect means of providing for oneself and/or one's family. In terms a human might understand: consider the hunter, the farmer, or the soldier. The hunter kills for food directly, his or her prey becoming sustenance with no internment. The farmer, either of plant or animal or both, will eventually kill in order to reap his crops, selling his excess to those who do not desire to kill. The soldier kills for a paycheck; which he will then use, ultimately, for self-preservation.
The second class, not by any means uncommon, will kill for desire. This animal will kill in order to attain something which is not immediately available to it without the death of its victim. This classification is extremely broad, and the actions the animal take must be considered even down to the most abstract philosophical level to designate it as such. For example: a thief may kill during a heist, a lustful young man may kill for sexual gratification at the petition of his object of desire, or a mercenary might kill for something arbitrary like glory or "honor." None of the rewards for killing, to this class, are a necessity for survival, yet they are for the indulgence of the animal.
The third, one belonging almost exclusively to humans, will kill in the name of something. This type of animal is complex and ignorant; a paradox, I know. The third class may be viewed as a subgroup of the second, in a manner of speaking, the primary difference being that the animal is oblivious to its actions being self-indulgent. The creature will fight on behalf of a belief or a person; for the honor of their king, their god, their creed, or a master. This being believes itself to be a proxy; those they kill are "not" done for their own sakes, but the telegraphed will of the one they believe they fight for. What these creatures fail to see is that their actions are, indeed, selfish in nature. They seek the adulation of the one they fight for. Ultimately they are killing to accrue favor or something along those lines.
Now, the next two classes of killers are a peculiar mix. The fourth and fifth animals are broken; they are a result of a shattered nous. Insanity, overbearing hatred, malicious intent, uninhibited desire; these are but a few of the origins of these two types of creature. They walk the razors edge between the natural world and the artificial one.
The fourth animal will kill for the sake of killing. Carnage, gore, blood, agony, and the creams of their prey; this animal is the unsuppressed sadist. Many of these animals have peculiar fetishes; the scent of blood arouses them, the thrill of the hunt is exciting, the pain inflicted upon their victims is luscious, among many others. They are devoured by hatred and insanity. Death consumes their every thought. The vast majority of these animals are short lived. There is a fundamental imbalance in the chemicals in their brains, which in turn inhibits their hold on reality. Often times this class of animal suffers from overstimulation, and their only reprieve is slaughter. Paranoia and hallucinations may lead them to mass murder, yet this also makes them extremely fragile because they are incapable of appropriately discerning threats. An animal of this classification will have a very narrow field of vision when presented before the object of their desire, and this almost always ends in their demise.
The fifth classification, by far the scarcest and, in my opinion, most frightening, also kills for the sake of inflicting death. The sole difference between the fourth and the fifth, however, is the fifth's complete lack of empathy. Even the most psychotic killer experiences emotion as they are slicing into their victims, though what these creatures feel is the opposite of what they should. This is not a weakness the fifth shares.
Why is that a weakness? Because the fifth, unlike the fourth, can control their instability. They are more rooted in reality than the fourth, and as a result they are capable of living much longer lives and killing with a much greater efficiency. The fourth killer suffers greater delirium during the process of slaughter and, as a result, breaks their ties to reality. It will revel in its own self-manifested ecstasy. The fifth experiences no such great thrill. Instead, the animal will murder with empty eyes and sharp senses. They possess the innate ability to blend in with society.
For what purpose does the fifth kill, then? If he derives no pleasure from death, then why is it that he professes such a vocation? I have often excogitated this, myself, and it is a difficult question with no singular, simple answer. The only thing I can say for certain is that absolute apathy is not, in fact, what these beasts experience. Rather… It is melancholy.
There is no better way to explain these animals than with that single word: melancholy.