<center><img src="https://i.imgur.com/bLa11UB.png"/></center>
Because an unprotected animal pet is a substitute for a human being.
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In many cases, these are usually the dogs, but I also saw many cases of the horse cruelty, and even cats and bunnies.
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I will assume you know nothing about dark tetrad so I will explain in short. <br/>
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Dark tetrad is a cumulative name for the personality disorders that include psychopathy, sociopathy,   narcissism, and Machiavellianism.<br/>
For some reason, you will find online resources that instead of sociopathy lists sadism instead and a sociopath is equalized with a psychopath, which is completely incorrect. 
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Let me show you why, in my own words. I am sure you will be able to find much better analyses in other resources.
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Personally, if a person is no direct threat in a violent way for the community, I tend to believe that anything that is 'born' is some sort of the evolutionary adaptation to the terms of the life.  
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Sociopath is made, <br/>
has the ability to feel complex emotions, hate, and love, <br/>
inclined to violent crime, <br/>
feels remorse for a crime, knows he done wrong,<br/>
human hater, <br/>
violent, unpredictable, erratic, rational, <br/>
lower IQ, <br/>
can suffer psychosis,<br/>
frequent and the long rage attacks, <br/>
impatient.
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Psychopath is born, <br/>
doesn't have the ability to feel hate and love, <br/>
not inclined to violent crime, <br/>
doesn't feel any remorse after a wrongdoing, believes it was necessary, <br/>
emotionally inept, <br/>
cold, calculated, logical, <br/>
higher IQ, <br/>
can't suffer any psychosis, <br/>
A displayed 'rage' is usually a very short act, 
patient.
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Narcissist is made,<br/> 
has the ability to feel hate and love, <br/>
not inclined to violent crime, <br/>
doesn't feel any remorse after a wrongdoing, believes he is entitled, <br/>
emotionally shallow and selfish, <br/>
emotionally unstable, <br/>
visceral, random, irrational, <br/>
average IQ, <br/>
can suffer psychosis, <br/>
usually the passive-aggressive display of the rage.
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Machiavellian is born, <br/>
has the ability to feel love and hate, <br/>
heavily inclined to non-violent crime,<br/>
doesn't feel any remorse, abuses all means for own gains, <br/>
emotionally calculated, <br/>
emotionally stable, <br/>
mental, organized, rational, <br/>
higher IQ, <br/>
can suffer psychosis, <br/>
no display of rage, manipulative. 
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<h2>Animal cruelty and mental disorders</h2>
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A sociopath is, in essence, a person who hates humans, but it doesn't mean he hates animals. Still, it is not unusual that in the absence of a more suitable victim a sociopath will turn to the violent rage display against the unprotected animal. 
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The book example is the dog owner who tortures the animal after having a bad day.  The most gruesome detail is that sociopath knows what he is doing and he knows that it is wrong, but he sadistically enjoys in violence and cruelty and later he also masochistically enjoys in self-deprivation and guilt about it.
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A psychopath is a person who lacks the complex emotional dimension and he is in no way inclined to harm either humans or animals unless they are considered dangerous. I will stress the word 'UNLESS' because it includes kind of a very loose and wide range of what we believe to be dangerous.
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Book example is killing any kind of a hostile animal without any feeling of remorse for the act. If the animal poses a threat, he will go after it in the most cunning way. The poison is a usual way because it leaves a little to no trace.
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The narcissist will use an animal pet as a platform to display the whole rage of the falsity of their character. 
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Book example is overexaggerated and usually false love and attention for the animals in order to gain attention and build an image of himself as a tender and caring person. 
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Machiavellian can't care less. If an animal is adding nothing to their personal interest he will probably give it to the shelter, or better yet give it to somebody else.
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Book example is a dog owner who finds out that his so-called blue blood champion is, in fact, some street breed so he gives it to the willing victim praising an exotic dog breed nobody heard off.
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Beneficiaries:<br/>
@woody26 ( article idea)<br/>
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