(All non-footnoted information was gained from an interview with my school guidance counselor with experience in Psychology.)

What makes someone commit murder? It is important to understand that there is not one specific gene, hormone, motivation, or childhood experience that makes someone commit murder. It is a very complex topic and there are many different factors that depend on context. There is also a difference between a person who kills one person versus a serial killer, and killing someone could be due to self-defense. Similar to the idea of a one time murderer versus a serial killer, homicide cases are not always based on the killer’s enjoyment or need for power or attention. Some people kill for revenge or out of anger (like if two people have a falling out, such as friends or romantic partners). There are situations in which murder is impulsive and not planned. Although this is the case, experts in Psychology have found that there are often certain patterns within people who murder out of enjoyment that help us to further understand what led them to their actions. (Click on photos for more information!)

Environmental Factors

A key factor of why somebody may kill someone is the environment. Environment can greatly influence people’s decision-making and behavior. Something that is crucial to child development is a healthy and safe family environment, and a dysfunctional family can harm a child’s brain development and therefore hightein their chances of criminal behavior.
There are certain risk factors, such as poverty, education, parenting, and family structure. Children who have a safe and nurturing home life are less likely to develop psychological problems that could lead to criminal behavior and therefore possibly murder. Children are 50% more likely to develop antisocial or criminal behavior if they were abused or neglected growing up. If a child deals with dysfunction and aggression with their family, they can believe that behavior is normal and mock that aggression. Oftentimes, these children live in dangerous neighborhoods, causing them to witness and experience violence often.
Exposure to violence as a young child can cause mental and developmental issues, which increases their chances of substance abuse and risky, suicidal behavior. If a child lives in a stressful environment and has to deal with issues such as an abusive or neglectful family and a dangerous neighborhood, the chronic fear and anxiety mixed with learned aggression can shape who they are as an adult.[6]
For example, serial killers are known to have experienced severe trauma, abuse, and neglect in their childhoods, which impacts brain development, emotions, and relationships with others. As children, they may have felt abandoned or less empowered, and killing can feel empowering.
Murderers or, more specifically, serial killers often grew up feeling alienated and left out from their peers in social settings. As children, murderers are often said to have been outside of what is considered socially normative or “outsiders”. If a child is bullied or isolated from other students at school, they can internalize their loneliness, which can turn into anger and cause them to kill someone.
Some people may take their anger out directly on the people that isolated them. This is common among school shootings where a student who has been bullied by their classmates may kill their bullies in order to get a sense of revenge. Social rejection can explain why people will inflict violence upon the environment they did not feel accepted in.
Many serial killers are motivated by thrill and enjoyment. They may find killing exciting and freeing, which can stem from the problems of neglect and abuse. Feeling in control and doing something enjoyable can act as a coping mechanism.

Neurological Aspects & Genetics

Although the psychological aspect of why people kill is greatly important to understand in order to be involved in law enforcement and the FBI, it is also important to understand certain factors like genetics and neuroscience. Genetics and neurological factors can also influence behavior, so being aware of them can help to understand a person and their actions.

A man named Jim Fallon, an American neuroscientist, was exploring the minds of murderers to see if there were any patterns of anatomical differences in the brain. He noticed that every psychopathic killer he looked at had damage to their orbitofrontal cortex, which is right above the eyes and controls decision making, reward signals, emotions, and impulsivity. Damage to the orbitofrontal cortex can make a person much more impulsive.

There are other neurological differences present in serial killers’ minds, such as a 5-10% reduction in grey matter around the limbic system, the place of processing emotions. The size difference is even more present in the amygdala, which are small glands directly connected to empathy and the regulation of fear. People with undersized amygdala tend to lack empathy. Another part of the brain that serial killers have damage to is the frontal lobe, which is the region for inhibiting inappropriate behavior, planning, and regulation of high emotions. Damage to this area can cause a lack of empathy, increased impulsivity, and violence.[7]

The major violence gene (the “warrior gene”) is called the MAOA gene, which has a variant that is in the normal population. People who possess this gene are often more aggressive. This gene is sex-linked and on the X chromosome, meaning a person can only get it from their mother. This can explain why mostly men are psychopathic killers due to the fact that they can only get X chromosomes from their mother, while women could get X chromosomes from both their mother and father, which would cancel out. In order for a person to express this gene in a violent way, there needs to be a combination of both having the gene and something very traumatic happening to them before puberty. So, if a person has the MAOA gene, it does not necessarily mean they will be violent, but if they additionally experienced trauma at a very young age, the risk is heightened.[8]

Damage or lack of activity in certain areas of the brain controlling emotions, empathy, and impulse control can be directly related to the behavior of serial killers. Because their brain cannot properly control these things, they act accordingly. The possession of the MAOA gene in combination with a traumatic event before puberty can be a dangerous combination, causing an increased chance of criminal behavior due to exposure to violence and the ability to carry out violence.[9]
More articles discussing the minds of murderers/psychopaths:

Symptoms, Traits, & Process

One of the psychological aspects of criminal profiling is identifying and understanding concerning behavior that is often present in murderers. Many killers are sociopaths, which is a personality disorder and mental health condition in which a person shows lack of regard for others, manipulation, lack of empathy, and ignorance of consequences. They are also sometimes psychopaths, which is similar, meaning they are unemotional, have a lack of empathy, manipulative tactics, and poor decision making. Once again, these people are often considered quiet “outsiders” and therefore rejected from a community. There are stereotypical signs that a child may grow up to be a serial killer, such as killing animals, rebelling, and impulsive behavior.

The truth is, a child can be all of these things and still not grow up to kill anyone. Some people may have been expected to become murderers, but others haven’t. Interestingly, sociopaths also tend to be endearing: they can be sociable and have the ability to engage in relationships, intimacy, and trust. Some murderers can be completely unexpected. They may show no signs of risky behavior or violent and manipulative tendencies. For example, John Wayne Gacy, an American serial killer who raped, tortured, and murdered at least 33 young men and boys, was a respected man in his community who would go to children’s parties dressed as a clown for entertainment. Nobody expected him to be engaging in these brutal killings. Some people are able to mask their symptoms and signs that they are going to murder someone.

So, what are the symptoms and processes leading up to a murder? It is true that during a person’s whole life, they could have been quiet, isolated, anti-social, and possibly violent and manipulative. These are sometimes people that engage in risky and carless behavior, insinuating that they are more impulsive and have less regard for consequences of actions. The FBI has done research on prior active shooters to understand and prevent future mass shootings. Mass shooters tend to exhibit less sociopathic tendencies in the time leading up to the shooting, possibly feeling pleased with what they are planning on doing. Some murderers will plan their killings, like a mass shooting, while others could happen impulsively. Often, someone who is planning to possibly kill someone is fighting through what is right and wrong. They are going through a difficult thinking process, which can make them more irritable and upset.

Serial killer profiles can consist of disruptive and violent behavior. From violent behavior can stem curiosity, and a person will wonder what it would be like to kill someone. This curiosity can turn into a fascination, and, eventually, an urge. In a way, it has to do with addiction. In some cases, a person may not necessarily want to kill but feel like they have to. It’s an urge and curiosity, which facilitates an action. The urge can be all-consuming and all a person thinks about, which can lead them to do it. They may plan it out precisely and pick a person, time, place, and plan to carry out a murder. They also might just be walking down an empty street and see someone who looks weaker than them, which is an opportunity to fulfill their need. Serial killers select their victims based on availability, vulnerability, and desirablity.[10] As I have said, the need to kill could be out of fascination and curiosity, but can also be or turn into enjoyment. If a person kills someone once, fulfills their curiosity, and enjoys it, they may do it again.

Ted Bundy, an American serial killer who had an unfortunate childhood

6. Hiromoto, “How Can Early Environments Influence Future Criminal Behavior?,” Godoy Medical Forensics, April 20, 2022, https://godoymedical.net/how-can-early-environments-influence-future-criminal-behavior/.

7. Ellison Cooper, “Neurology of Violence,” Ellison Cooper, July 16, 2019, https://ellisoncooper.com/neurology-of-violence/.

8. Joseph Stromberg, “The Neuroscientist Who Discovered He Was a Psychopath,” Smithsonian (Smithsonian.com, November 22, 2013), https://www.smithsonianmag.com/science-nature/the-neuroscientist-who-discovered-he-was-a-psychopath-180947814/.

9. Stanford Iranian, “Robert Sapolsky: The Biology of Humans at Our Best and Worst,” YouTube, June 12, 2018, https://www.youtube.com/watch?v=GRYcSuyLiJk.

10. Scott Bonn, “Understanding What Drives Serial Killers,” Psychology Today, September 15, 2019, https://www.psychologytoday.com/us/blog/wicked-deeds/201909/understanding-what-drives-serial-killers.