I never liked explanations containing going crazy. The whole abandoning yourself is classic "become emotionless psycho". But Marika just seems to do random shit.
Other than the events of the shattering, which are presented as a mystery there seems to be clear cause and effect to her actions.
>Everybody in her life dies so she wants to take vengance on the people who killed them and prevent anybody else from having to lose loved ones.
>Goes about acquiring the means to enact above said plan.
>Fuck, kids are cursed
>Don't want to kill them but can't stand the sight of them and more and more people are being born cursed, their blood literally antithetical to the your "perfect" world.
>Create shunning grounds
>Gloam-Eyed Queen(Melina) and God Skins attack in order to return Destined Death
>Erdtree gets partially burnt down and Godfrey is replaced with Radagon.
>Sometime after this exile Messmer due to his snake powers becomes a target of Marika's paranoia, he might alsodo what his younger sister did since he also has visions of flame.
>Shattering happens and Melina's amnesiac spirit is sent out to return Destined Death and try to fix the golden order.
On that last part, I don't think Marika has a solution, she's hoping the resurrected tarnished have one as Goldmask(Pefect Order), Fia(Death-Prince), Dung Eater(Curse), Gideon(Fracture), Godfrey, and the player are all guided by her grace.
Maybe the Hornsent are different. I never got the impression that they naturally grew horns, only that they fetishized them as a symbol of divinity. Perhaps I missed that they did grow them.
It's mentioned in items descriptions that growing too many horns too fast can kill Hornsent, especially babies, but they aren't in pain and only grow horn on the head.
Compared to the Omen who horns grow cancerously across their whole body, the ones we do see that have weird horns are the Curseblades, who are cursed. Those spots where you get spirit ashes from are the Tutelary deities who Curseblades try to become and the ones that haven't been beheaded, indeed have a mass of horns for a head. The only other Hornsent with weird horns are the Empyrean Grandam, who's very old and magically powerful and the Hornsent elites who have those horns as part of their masks.
- The hornsent used to rule the Shadow Realm which either used to be part of the Lands Between or is the realm of mortals which is literally beneath the Lands Between (in the Shadow of the Erdtree™).
- They hated the shamans (Marika's people) for some reason (denying the divinity of the crucible?) and decided to persecute them and punished them by shoving them into jars.
- Marika conned the hornsent into building a magic tower that allows one to become a god (and possibly leads to the Lands Between?).
- After becoming a god and conquering whoever used to run things (Gloam-eyed queen? Placidusax?) with Godfrey at her side, Marika sent Messmer down to the shadow realm to take revenge upon the hornsent and subject them to the same sort of torment that the shamans suffered through.
- After Messmer's horrible Dracula-esque persecution, the
people of the fishing village hornsent put a curse upon the Hunters Marika and the residents of the Lands Between as a whole that made some of their babies grow freakish horns that caused them immense pain.
Messmer came way later to the Lands of Shadow as he was close to Rykard, his army makes use of Rykard's abductor Virgins which were made by Rykard, and Omen Killers are in his army as well. So it's likely he was sent out in response to the Omen curse or the Gloam-eyed queen's attack making Marika more paranoid of him.
Also I think we often fail to think of how long ago the events of the shattering were, but long enough that Godwyn's knight reforged all of their armor and weapons to be in the Death Prince style, and also take up Death magic.
Messmer's followers also only learned he was part serpentine after he moved to the Lands of Shadow as indicated by the Spirit Ashes of Huw and Andreas, meaning they also reforged all of their gear to have snake themed and have to had time to develop unique snake themed weapons.
But on the subject of the Shadowlands, it was always part of the Landsbetween before Marika removed the rune of Destined Death as indicated by the tower of sealing which says. "The very center of the Lands Between. All manners of death wash up here, only to be suppressed."
Here's a map I scribbled on and some concept art to help you visualize it, althougth the scale and size of shit is fucked in the in game map due to vertical distance being shown horizontally. The concept art map is also missing Dragonbarrow, but you can see the end of the ravine that begins in the Mountaintops like you can in game.