I’m afraid you’re completely mistaken, believing in 19th century style textbooks written in a way to show that our current, modern situation is much better and comfortable. Even today’s remaining hunthers-gathererers who cook some of their food aren’t that aggressive and enjoy a life much better than ours.
Even Wikipedia https://en.wikipedia.org/wiki/Paleolithic#Human_way_of_life shows a much more balanced view and describe a most often peaceful paleolithic way of life.
Like I said, plenty of mainstream archeologists foster the myth of the noble savage, whereby any social behavior that H-G tribes engaged or engage in is inherently good. They either ignore or fail to report the horrors of uncivilized life, often because they believe it's not up to us civilized people to judge the savages for their ways, because morality is seen as relativistic and what we see as bads in our culture aren't necessarily bad in theirs. Part of what drives this bias is an embellishment of their field of work. Clearly they can derive much more funds and interest in their research if they depict the primitive life as some sort of paradise. Look up psychohistory and their stuff on early infanticidal childrearing cultures for an opposite view.
But like I said, research like this is not even needed. I can show you why primitive tribes must've been brutal by just using economic and sociological/psychological theory. Like I explained, resource availability in a H-G environment is mostly fixed and can't be increased, resources can only be extracted. As such tribes must form to defend a territory and keep competing tribes at bay.
Within the tribe, individuals must be forced to sacrifice their own genetic welfare for the good of the tribe. But because all lifeforms are genetically selfish, this must be beaten into them. That is, if a tribe member acts in a selfish way, whether extracting resources from the land without sharing, not extracting enough resources and simply leeching off of the rest of the tribe, or giving their resources only to their own offspring, they face punishment by the rest of the tribe, including the very real possibility of death. Another act of selfishness would be if a tribe member refuses to allow their own offspring or sibling to be killed when the tribe deems their presence unnecessary or harmful for the tribe. Since human breeding produces far more babies than the amount of naturally available resources can sustain, rampant infanticide and the murder of toddlers and young children are used for three reasons: 1) to keep the population in check, 2) to select for future tribe members that will fill the roles most needed by the tribe at the moment, and 3) as a threat to induce compliance and self sacrifice for the good of the tribe
As it regards other neighboring tribes, like I mentioned, they must be regarded as mortal enemies, since they're extracting resources from the same overlapping pool, and thus any resources they extract necessarily means your tribe must reduce it's population by killing more of their members. Furthermore, both tribes know that as soon as the other is able to, they will wage war and attack them, attempting to either kill all of them, or in the case that there is enough land available, they'll kill all the men, and come back repeatedly to rape all the women, thus spreading their genes onto the other tribes' genes, facilitating the making of babies at a faster pace than their own tribe alone could produce. This creates a necessity to have most men in the tribe have a psychological need for war, to become bloodthirsty warriors who revel in the killing of enemy tribe members, and to enjoy raping enemy tribe women as the spoils of war. But war has risks, and except in extreme cases, it isn't genetically worth it for an individual man to go to war like this, so again, their psyche should be shaped by the tribe in such a way that it becomes twisted and they can only find peace of mind in engaging in these acts. The way you do that is to constantly torture and rape or sexually assault boys since they're born, and to further teach them to rape little girls in their own tribe as they begin to grow up. Also, the constant threat of murder if they don't comply with the tribe's wishes helps here, and the watching of their own siblings and other tribe members being mutilated, murdered and eaten in front of their eyes. According to psychohistory researchers, this is most likely what creates the schizophrenic mindset, with most schizophrenics today reporting to hear voices, and the voices telling them that they're going to murder them, the typical paranoid schizophrenic being in constant fear of being murdered, by their caretakers at first, and by larger institutions as they grow up. Similarly, girls must also be constantly tortured and raped since they're born, and watch all the horrors in the same ways, both to instill compliance, and to teach them how to engage in childrearing as they grow up, so that they too need to find peace in torturing, mutilating and sexually assaulting babies and young children, in particular the little boys. Also, to accept that being raped is a part of life and that they should not put up too much of a fight and just let it happen, which will increase stability within the tribe, and also possibly save their lives and ensure some genetic reproduction in the event that their tribe is conquered by an opposite tribe. Girls are encouraged to let the tribe abuse them in whatever ways they see fit, and in turn discharge that abuse into the little kids in the tribe, to ease their suffering.
Let me just give you one example of such a thing:
"Consider a typical infanticidal, incestuous culture, the Bimin-Kuskusmin of New Guinea. As is so often true in pre-literate cultures, the mothers have long post-partum taboos against sex with their husbands, sleep naked against their children until they are about four years old, have orgasms while nursing them and regularly masturbate them. One three-year-old boy describes how whenever his mother was sad or angry she masturbated him so roughly that it hurt him, and he struggled to get away, complaining of a pain in his penis. “It hurts inside,² he told the ethnologist. “It goes Œkoong, koong, koong’ inside. I think it bleeds in there I don’t like to touch it anymore. It hurts when I pee…² Sometimes, after his mother hurt him while masturbating him, he wounds himself in the thigh and abdomen with a sharp stick and draws blood, looking at his penis and saying, “Now it hurts here, outside, not in penis. Look, blood. Feels good…² Although he is only three years old, he understands quite well that he is being used as a poison container by his mother to relieve her depression. He says, “Mother twist penis, tight…Hurt inside…Mother angry, hurt Buuktiin’s penis. Mother sad, hurt Buuktiin’s penis…Mother not like Buuktiin’s penis, want to cut off…” " -“On Writing Childhood History.” The Journal of Psychohistory 16 (1988): 135-171.
Notice that none of these 2 main necessities apply under an agricultural setting, where each individual can increase the production within their own private piece of land without negatively affecting anyone else's (in fact increasing others' wellbeing through trade); peaceful cooperation and trade (relying instead only on defensive violence, but not offensive), private property, a private polycentric legal order, lovemaking based on seduction instead of rape, marriage and family unions instead of polygamy and tribal unions, and a childrearing mode where the child is helped to reach all of their potential, allowing them to retain their individuality and fulfill as much of their own goals as possible, not for the tribe's sake but for their own sake, all become much preferable in order to maximize production and standards of living. The problem is that it takes many, many generations to shift from one mode to the other, apparently opposite one. And there is reason to believe that until very recently, nobody had even figured out what the optimal social organization was for the agricultural setting, or to the extent that somebody did figure all of these things out, their ideas did not become known. That said, we can see a clear transition through the millenia in favor of these more civilized ways and away from the barbarism of our ancestors. But like I said, there are periods where we turn back and let our genes and instinct guide us, and that's where we go wrong.