There's never a satisfying answer to the question of why humans behave as they do. Science, philosophy, and religion all have their own positions on the matter. Naturally, they do not align.
The underlying pursuit is for whatever might constitute "human nature." Whoever can crack that code would, it is assumed, be able to determine the future of every individual. Everyone would have a script from which they would not deviate. With human nature fully understood, it could essentially be navigated to whatever ends those in power desired.
But the question of human nature isn't as vague as the phrase makes it sound. What we really wish to know is: are humans naturally violent, or naturally cooperative? Is our underlying psychology one of competition and dominance, or are we more predisposed to friendship and collaboration?
To answer this question, the scientific approach is to examine our closest relatives: other apes. Chimpanzees, in fact, are known to revel in brutal violence against one another when nearby communities are in competition--usually over females. Some of the actions male chimps take against competitors would be grave crimes if committed by humans; if classified as warfare, they would almost certainly invoke charges of war crimes.
The studied behavior of chimpanzees indicates that, when they savagely murder one another in warfare, it is not simply a utilitarian act. A male does not tear the scrotum from another male out of pure survival instinct, operating from an understanding that depriving this male of his virility will improve the killer's chances of passing on his genes. The reward is not the promise of copulation and progeny--the victorious males receive that reward as a matter of course, and it depends not at all on how grotesquely they dismembered their foes. Rather, the sense of power that emerges from the act of killing seems to be the reward. In other words, the ape brain is genetically programmed to enjoy the infliction of senseless violence for its own sake.
It could be easy to assume, then, that the human predilection is toward violence--undirected, pointless violence, at that. But humans are not alone in this! Predatory mammals, such as cats and dogs, are known to kill for sport. They will torment and torture a prey animal for an extended period, seemingly enjoying the victim's struggle and pleas for help. When their amusement has reached its end, they will kill the defenseless creature and leave its carcass untouched. Whatever pleasure centers in the brain light up from the act of murder were not gifted solely to the apes.
Dolphins, too, kill apparently for the fun of it.
The matter is somewhat more complicated, however: interspecies killing is commonplace, even without an obvious survival purpose. But what about homospecies slaughter? About 5% of all mammal species are known to kill their own kind, including humans. Still, does this tell us anything helpful?
There are many reasons to kill, after all: self-defense, the promise of a mate, competition over food and territory, establishing and maintaining hierarchy within pack structures, and the list goes on. Less certain is how much intraspecies killing lacks any more broadly explicable motive, meaning it was done solely for the pleasure of the killer. Other animals cannot tell us, after all.
If nothing else, we know that other apes do it. What humans have that other apes don't, however, is the capacity to formulate and debate ethics systematically. Other apes have norms, certainly. They engage in ritual behavior, reinforce conformity and punish deviance. There is little question that they also possess culture, in the form of gestures and practices that are unique to specific groups. But there is scant evidence they are capable of longitudinal analysis of their behavior, either individually or collectively. Only humans have laws, courts, and jails. Only humans write voluminous philosophy in search of the inner heart of human nature. Only humans devise ornate systems of belief to explain the machinations of the Earth and sky, and then spread those beliefs to others, at the point of a sword if necessary.
Taking the above, it would be easy to assume humans are not that far separated from the apes; we have simply embroidered more complexity around our rituals of competition and killing. Had humans never endeavored to settle together in huge numbers, that argument might be compelling. But one way humans differ vastly from our ape cousins is that we developed the capacity to densely cohabitate. No other mammals take up living shoulder-to-shoulder by the millions, certainly not voluntarily. That by itself should put the lie to the notion that humans are fundamentally, incurably violent.
This is not to say that violent impulses have ceased to exist. We have ample evidence to the contrary. But where other animals are ruled by instinct, humans have the ability to premeditate and contemplate our actions before we take them. We live together mostly peacefully, even increasingly peacefully. The obvious question is then: isn't this all just a side effect of surplus food and supplies? If our modern amenities vanished tomorrow, would humanity rapidly deteriorate back toward an endless rampage of wanton misery?
In crises, humans are more apt to help one another than behave as post-apocalyptic highway bandits. There may be some who romanticize the latter and believe it is the key to their survival, but in point of fact the only thing that has ever ensured the survival of humans is cooperation, built on trust. Much is made of the fact that apes are also social creatures. They are unhappy when solitary, prone to self-harm and stunted emotional development. They require social bonds, as do we. Humans obsessed with the notion of going it alone against the world defy all understanding of even the basic tenets of human nature. If there is to be a future at all, it will be built by people working together, not disparate loners.
All of this is perhaps beside the point. The deeper question is not how humans are prone to act, but why. Is there free will? Is behavior preordained by genetics? Complete answers would require data on prehistoric humans we are unlikely to ever acquire. Nevertheless, there are some insights to glean.
There is little question that human behavior is influenced by genetics, upbringing, and other people. The degrees to which these interact is unknown and likely varies from person to person. From the genetic perspective, the brain's sensitivity to the various neurotransmitters involved in aggression has a lot to do with how violent or peaceable an individual is inclined to be. It is not deterministic, of course. The additional factors play a large role, too. Upbringing could be expanded to include not just the process of being raised by one's parents, but gestational events and environmental factors such as pollutants. Proper prenatal and neonatal care can help ensure what one might call prosocial behavior. There is evidence that exposure to high levels of lead in childhood, such as from industrial waste sites contaminating local groundwater, can promote aggression, reduce impulse control, and harm intellectual development that might otherwise make an individual more contemplative than reactive. And then we come to the depths of hell, which is to say: other people. Humans are so prone to being influenced by one another that history records countless cases of otherwise seemingly reasonable people collectively participating in unspeakable horrors, simply because the notion of doing so spread among them and, perhaps, nobody wanted to go against the group, especially when those who did met with terrible fates.
It would make no sense to say that humans act completely out of an unbridled free will. We do not behave in a vacuum. We do not make choices without assessing, even in some automatic way, what the consequences might be.
And there remain peculiarities between humans that must influence the ways in which people interact with one another and the broader world. A small percentage of people cannot form mental pictures, for instance. Perhaps more intriguing: fewer than half of humans report having an internal monologue, in which they hear their own voice narrate their thoughts, experiences, memories, and so on.
The latter is fascinating because there is no other precedent we're aware of for an internal monologue in the first place. It could be assumed that such an inner voice requires an ego, an "I," to materialize it. But early human writings, particularly of a religious nature, suggest that humans were once prone to hearing voices in their heads that told them what to do, against which they felt almost powerless. Some may have called this voice "God," or conceived of it as other supernatural influence, or maybe even tried to ignore it. If that's the case, then somewhere down the line, perhaps only a millennium or two ago, the populations of humans who did experience an inner monologue began to integrate that voice directly into the ego, understanding it as the self rather than some invisible, outside influence.
Consider, now: chronostasis. Face clocks offer the most direct experience of this particular phenomenon. Locate a face clock with a second hand that moves in per-second increments rather than a continuous rotation. Avoid looking at it or thinking about it for a length of time, then stare closely at it. Watch the second hand. It may seem to stand still for longer than a second before you see it move. Why? Did you stop time for a moment? Of course not. Rather, your brain has lied to you.
Your brain is lying to you all the time. Your eyes have blind spots which are automatically filled in by whatever patterns your brain believes will provide the most convincing, complete picture. There are ways to find exactly where your blind spot is by tricking your brain into producing bad illusions.
Your brain receives millions of stimuli every second. Out of those, perhaps 12 or 16 can be raised to your conscious attention. This process is not quick, and produces a lag of about half a second. The lag can be greater if the source of the stimulus is far from your brain, as well. Pain in your foot, for example, takes slightly longer to register because the electrical impulses must travel from nerves all the way up your body.
What is your brain doing with all that lag-time, then? It is concocting illusions and making decisions for you. When the second hand on a clock holds still longer than it should, your brain is calculating whether the movement perceived by your eyes is genuine and that you would want to notice it in the first place. Once it decides such, the hand moves, then keeps moving as expected.
If you have ever reacted immediately to danger or an unexpected stimulus, this is the result of your brain taking over and acting on your behalf rather than waiting for a conscious decision to be made. One could imagine that consciousness is essentially a marionette on a stage; from the outside it may appear to move of its own will, interacting with its surroundings, but the puppeteer sees the entire scene and pulls the strings.
This makes the question of free will seem hopeless, and the purpose of an inner monologue to be something akin to mental masturbation. Extrapolated to human behavior more generally, one could assume humans are thus destined to behave out of automatic reactions, and those reactions then easily spread through populations, giving us religious crusades, genocides, protest marches, sensationalized media cycles, and so on and so forth.
But the puppeteer-brain offers the marionette one saving grace, reserved for the occasions of importance where an instant reaction is not required, and the option to deliberate is available: the veto. The brain has primed a course of action, polling the neuron pathways stimulated by the present situation, ready to act. But it does not simply act. It gives you a moment--maybe two or three--to say "no." Should you defy the predetermined action, you may find yourself briefly paralyzed, indecisive. There's no script for this situation--or if there was, you just disposed of it. You must now decide. That is the gift of consciousness. That is the element of human nature that sets us apart, that has gifted us the capacity to think and plan beyond the next few seconds.
And so we see, here, at last, the power of the veto, the authority of the "no."