It doesn't shatter anything in Keltham that wasn't shattered already.  Casting aside your Law is not hard.  He decides that he will, after some preparation, just shift himself to Neutral Evil.  It'll break his bond to Abadar, but he wasn't planning to keep it anyways.  If "Abadar" did not care to pay a little more and save him from this fate, he owes that god nothing; It intended to make use of Keltham, was all.

You cannot easily break a dath ilani's deontology just with ordinary trauma.  They are not made, do not choose to individually make themselves, to be easily broken; for most ordinary traumas they could just decide not to break, instead.  To call someone from that culture "traumatized" is scarcely more useful then calling them "insane", for purposes of filling out a detailed careful predictive model of precisely what they'll do next.

But there are sufficiently extreme and prior-improbable life experiences which will cause a dath ilani to reconsider whether their previous set of ethical injunctions are appropriate to their new environment.

The Project has not prioritized corrupting Keltham to evil, with Carissa Sevar not in charge of it; he knows what masochism is, and submission, but Carissa Sevar was instructed by authority not to push him into it and risk triggering his suspicion.

There's no plausible version of the story, in this iteration of the story, where Keltham's mistake as evaluated looking back was his being too tolerant of evils and short-term harms.  His mistake must have been—not even being Good, so much as, being himself.

The foundation of Civilization's second-order utilitarianism, as taught in their schools, is that—among other things—being cautiously nice isn't supposed to result in your loved ones being shattered in Hell and you having helped an endless-torture dimension to conquer your host planet.  Being cautiously nice isn't always supposed to result in the locally best outcome, maybe not even a good outcome, but it's not meant to result in that.  If it does, surprising you, maybe you were wrong about something.

(Though this Keltham will still, to the end of his mortal existence, never break any explicit oath that he has made.  He will never stab anyone personally with a knife, will not harm anyone in any way that would be effective for somebody inside a physicalist universe and hallucinating.  Just in case he's in dath ilan and insane and dreaming all of this as his brain's excuse to violate deontology.)

Carissa Sevar is less watched, on the Project, with Keltham out and gone.  There is less Security on the archduke's villa.  In time she has her mid-night revelation of her own folly...

...but this Carissa does not think at once of how Keltham might destroy Creation.  There was no Vision of Hell and no godwar and Zon-Kuthon has not been a subject much discussed; nor whether Civilization would delete itself to end Xovaikain if it had no other options.

(The thought of erasing her memory and selling her soul to Dispater doesn't even occur to her.  You need to have lived in a visibly trope-influenced universe for that to seem like the sort of thing that people get away with, and this Carissa Sevar hasn't been there.)

Her escape from the archduke's villa is more harrowing than in other timelines.  Osirion is already ruined and conquered by Cheliax, and anyone there with high Wisdom who could not flee chose to wisely die in battle; escape is not as simple as praying to Abadar and then killing herself to await resurrection.  This Carissa Sevar does not hold commanding authority on the Project, does not have Securities personally loyal to her in hopes of a kinder Hell.  She has not been tormented twice by Abrogail Thrune, and is not fifth-circle.  She did reach her fourth circle, in time, for she was casting and crafting under peril deeper in some ways than the Worldwound; but she cannot Teleport under her own power.

Carissa Sevar escapes anyways.  Irori does not bestow His attention lightly.

Keltham is not easily found, but he has taught this Carissa Sevar of blind-coordination focal points.  There should be a place that Carissa Sevar would think of after relinquishing Asmodeus, that Keltham would also think of.

Her first guess doesn't work.

Going to the ruins of the Iomedaen temple nearest to Keltham's arrival point by the Worldwound does.

From there, Carissa goes to meet her Keltham.

Of Wishes and artifact headbands she has none, nor the souls of those he once employed or loved; there was no visible way to obtain those, in this branch of reality.

All she has, is all she is; and if she doesn't offer Keltham that, it means offering him nothing.

And Carissa Sevar finds her Keltham, and falls to the floor at his feet; and offers up all she is, in sorrow and in penance, to help him in his plan against Asmodeus.  For she is sure, knowing Keltham, that he has a plan like that, and that he is not content to wait in despair.

By then Keltham has scryed Peranza and Asmodia's fate, knows that Peranza is already shattered in Hell beyond all hope of repair, that Asmodia in Xovaikain still remembers something of herself as she screams and screams and screams and sometimes calls his name.

Keltham of the latest iteration found it necessary to proclaim that he'd been effectively killed, destroyed, by his experiences.  The Keltham of the zeroth iteration doesn't have any need to say anything like that; it's just true.

Carissa returns to a man that she has already destroyed, whose love for her is shattered along with everything else about him that she loved.

This Keltham warns her not at all.  He accepts his fate, to betray her as she betrayed him.  Carissa didn't want to do it, but did; and Keltham does the same, because he has children in the Boneyard and sometimes Asmodia screams his name.

He binds Carissa by geas to honesty and promise-keeping—for he still has great sums that Cheliax by compact owes him, though he negotiated less aggressively in this timeline.

Carissa Sevar accepts that binding as part of what she thinks is her penance.

As her primary task, Keltham sets Carissa to crafting a +6/+6 Intelligence/Wisdom headband for himself.

Splendour would be improbable, as something that she could attain on top of that; and Keltham (though he does not say so) would not want it if she could.  This Keltham has tried casting Eagle's Splendour on himself, from scroll; and he found that +4 Splendour's balance of making his emotions stronger, versus giving him more force of will to endure his own emotions, was more painful than helpful.  He does not want his Splendour increased again.  It makes him more himself again, and he'd rather not be.  There aren't, really, much in the way of feelings that he wants to have.

Keltham's diamond-making takes longer.  He does not have kingdoms and empires offering to be his purchase-agents.

Keltham tells Carissa nothing of that diamond-making, before or after.

He goes to the City of Brass alone, and in secret, and augments only himself.  He doesn't augment Strength or Dexterity, except with a belt he can remove, because the improvement might be too visible.  He doesn't augment Splendour.

What returns from Brass is even less the same person.  He didn't want to be himself anymore, and INT 29 / WIS 27 / CHA 14 enables him to make a good start on that Wish.

The entity that returns from Fommok Madinah—which one might as well go on calling Keltham—does then a thing which intact dath ilani would not do:  He shows Carissa Sevar a Wish-scroll (that he purchased in Brass at ludicrous cost), and asks Carissa Sevar for her help with understanding Wishcraft.  Keltham doesn't name any particular Wish; he says he wants Carissa's analysis about what he can Wish for, and what that would take, and how it might go wrong.

She does not know, when he asks for her analysis, that he is INT 29 and not merely INT 24 like her.

The truth, Keltham has already deduced, is that Wish wordings often seem to be interpreted in a deliberately perverse way, as if some anti-genie were trying to minimize the caster's utility function subject to the constraint of the words spoken.  One can deduce that truth in some detail; there are cases where Wishes did, not harm in general, but something the caster particularly wouldn't have wanted.

You can maybe conjure an exact mass of an exact kind of antimatter in an exact place—if you use Wish-words carefully enough, for the length of the wording is also a conserved resource.

But if it were something more complicated than that, which an antimatter blast alone might not accomplish, you would obviously want to...

And INT 29 Keltham carefully filters which true sources of evidence Carissa Sevar receives to examine, which legends, which accounts.

Carissa Sevar concludes that the caster's conscious intentions are what a Wish perverts.

She herself proposes the clever solution that seems obvious to her, the solution that Keltham foresaw that Carissa would deduce from filtered evidence:  You might be able to get more done, if you'd done everything else right, if at the last stage somebody casts the Wish without knowing what the wording means.

Keltham gives Carissa her last instructions as she stands within a concealing darkness they have Teleported into, to read a Wish scroll whose Wish-phrasing is written in a tongue she knows not.

And by then she does have qualms, even though it was her own plan that brought herself there, but—

Trusting to Keltham's honesty, at the very last, she speaks the Wish-wording he gave her.

And the outcome that comes about is the one that Keltham needed, if not wanted: an outcome pessimal by that Wish-caster's own values.  For the truth was that Carissa stood unwitting far beneath the surface of Golarion, close to where the strange planar boundary of Rovagug's Dead Vault infringed upon the Material.

...And then, with the gods thoroughly distracted, Keltham destroys Absalom wholly in a single blast of antimatter that leaves plenty of safety margin about overwhelming Aroden's protections; and with a third Wish sends Achaekek to Its death; and touches the Starstone with his last mortal thought being: to fix this world or destroy it, bring Pharasma to heel, and tear Asmodeus out of reality at any cost.

The subsequent fight against Rovagug destroys Golarion, of course; but in time It is driven out of Creation before It has consumed more than a handful of other planets.