The simplest answer is the material conditions of having one's direct material interests conflict with other people's rights and survival. It's a sort of cognitive dissonance resolution thing, that when someone is benefitting from hurting someone else they'll start trying to rationalize it: it's their "right" to take this for themselves, it's the "natural order" of the world that they should receive and others must suffer for it, the ones who are suffering are wicked and deserve to be hurt, and so on. Give that sort of mindset generations to fester and stew and you get it formalized into all sorts of violent reactionary ideologies. This is true whether one's talking about bourgeoisie, landlords, privileged ethnic groups, men in a patriarchal society, or settlers.

The specific arrangement of settler colonialism is also effectively a trap: the settlers' entire way of life and continued comfort depends on the brutal order of extraction and theft continuing, because their homes are on stolen land (and sometimes are literally homes that were directly stolen as-is), because their jobs are on stolen land, because all their wealth is tied up in systems that rely on the settler state. This means that an end to the settler project and a redress of its crimes means they lose everything and are destitute in a reactionary system that unpersons anyone who lacks land and wealth, that they would become refugees and find themselves at the mercy of other predatory capitalists eager to exploit and destroy them for profit. Considering that engaging in a genocidal settler colonialist project in the first place means starting from a position of already having a genocidal supremacist ideology, you can see how it only gets worse with time as the material conditions make it entrenched and more costly for its perpetrators to stop.

Edit: fuck this is reminding me of a "textbook" on military strategy I read when I was in highschool and was the sort of insufferable nerd who'd read stuff like that, which apart from having galaxy brain takes like "partisan resistance movements are a bad strategy because 'they are not militarily useful' and further 'corrode social morality' leading to restless populations later" also literally talked about and praised settler colonialism as a method of conquest for rather similar reasons to those I'm condemning it with here, that the settlers necessarily must be in conflict with the natives and so would "be more loyal" to the core that they both rely on and have cultural ties to than a subjugated native population would be. I looked around to see if I still had it so I could look at its brainworms through the lens of marxist analysis I have now, but its not on my bookshelves so I think it must be one of the ones that got ruined or lost while I was moving. I'm just struck by the memory of how it was basically taking a correct material analysis of some things and then applying the most ass backwards moral valuations to it to the point that now I'm wondering if it wasn't written by some trot turned neocon or something.

in other forms of colonialism you can profit from the natives being robbed. Settler colonials however require the natives to die so the claim to their land can be fully realised.

actually they need the natives to have never existed in the first place but the first step to that is to kill them

this was one of the sources of conflict between Britain and America in the run up to the American revolution

They fear the violence that they inflict will be turned on them. Their one actual ideological position is that the violence must never stop and always be aimed at the colonial subject

It's because settler-colonies are inherently genocidal. The settler-indigenous dialectic functions different from the bourgeoisie-proletariat dialectic or the aristocrat-peasant dialectic. Those other dialectics can be generalized as a master-slave dialectic where the master oppresses and abuses the slave but is still reliant on and can't truly get rid of the slave. The settler, unlike the master, doesn't need the indigenous. In fact, the settler needs the indigenous gone asap in order to steal their land and other natural resources. Therefore, the settler-indigenous dialectic can be resolved not only with the overthrow of the settler-colony by the indigenous like all oppressed groups overthrowing their oppressors, but it can also be resolved with the extermination of the indigenous at the hands of the settler. This inherent genocidal goal of settler-colonial society leads to settlers upholding genocidal ideology and in general, acting like deranged monsters.

They can't give an inch. The settler state is a Jenga Tower of lies and violence that could be toppled if too many pieces are removed. So it must be constantly reinforced with more and more lies, more and more violence.

Fundamentally - There are people everywhere, and there have been for tens of thousands of years. We keep pushing back the date when people arrived in NA (per European science, Indigenous people have their own accounts). There are only a handful of landmasses in the world that haven't been settled for at least 1k years or more.

So, if you're going to colonize and settle somewhere there are already people there, and they're in the way, and you're going to have to do incredibly cruel and violent things to take their stuff and exterminate them.

Racism, yes. But also most settler colonist states have a history of trauma that drives the actions of the settlers.

The US and the Puritans, New Spain and a bunch of PTSD'd child soldiers fresh from the reconquista, SA and the Dutch refugees, Convict Australia...etc.

The people colonial powers use as settlers are traumatised into leaving by the power, and that trauma weaponised against the existing population. And that traumatic superstructure sticks around for centuries as a mindset of "only oppession is possible."

common thread seems to be religious fundamentalism.

I’m gonna go out on a limb and say it’s the settler-colonialism

It boils down to racism. When you internalize the stupid racist theories, it is even justifiable and beneficial (in their twisted worldview) to slaughter/enslave them.

I almost feel like settler colonialism is just an extension of a deeper evil, which of course begs the question of where that evil comes from, but I think settler colonialism is just "we want to take your stuff and live here, and will justify that violence to ourselves in whatever way we can". And then those justifications have to get more and more dehumanizing the more land you take and the more indigenous people you slaughter, at which point the base steers the superstructure, and then the d i a l e c t i c shifts, and the superstructure starts steering the base. And then there's the whole issue of the massive discrepancy in military firepower, then with muskets but now with bombers, which means you never have to really be confronted with a proportional response and even start considering peace, you can just keep gunning.

I want to build a house here
that means these people who live here have to be removed
guess I have to use force
but that's okay because I will make better use of the land than them
plus they are probably subhuman anyway
no wait they're actually definitely subhuman
otherwise they wouldn't have attacked us peaceful pilgrims
maybe we got a little too angry and perhaps overreacted when torching their village
but in retrospect they would have killed us if we hadn't struck decisively first
I bet the neighboring village will have an emotional reaction upon hearing this
barbaric savages that they are
so it's for the best that we strike swiftly against them too

TL;DR: They wouldn’t be settler states if they weren’t.

You said it, its the blatant dehumanization, people are seen as being innately lesser in settler states by settlers so they're 'just part of the environment' like a forest or a crop of grain (hence all the garden of civilization bullshit by colonialist sorts), and this is used to exploit them in a multitude of ways. Add on capital accumulation and yikes.

When your dog barks at you or your cat scratches you, most people would be a little offended. You’re taking care of them, making sure they eat and have a warm bed, and they lash out on you.

This is how settlers see the colonized. You’re not a human, you’re a domesticated animal who needs to be trained. And if you lash out against their generous civilization, they become offended.

Is colonialism a symptom of whiteness or is whiteness a symptom of colonialism? I don’t know which one came first but one of them of them is the symptom and the other is the cause. We need to find and treat the cause.