How do you think hiccup will react for when Tom tells him about his battle with Jormungandr in the ancestor au?
You know what? I actually don't know. That takes figuring out just why Hiccup decided to trap Jörmungandr in the first place and I haven't been able to figure it out. Other than, you know, The Nine Realms being predominantly for a younger audience, which means "hurting animals/other people bad" when other instalments were for older audiences and could therefore allow a little more nuance.
Like how the Red Death did need to die after causing almost 400 years of bloodshed on both sides and for being both directly and indirectly responsible for a mountain of corpse. Like, Stoick worrying in Httyd 1 about feeding his village after another dragon raid made off with most of their food source. With the battle happening with all the Vikings and dragons there at risk of her wrath. Or like how defeating Drago's Bewilderbeast was indeed the way to go as there was no room for a gentler option. Not with Drago, his abuser, right there or with the Bewilderbeast actively attacking Berk and almost killing Hiccup, which is ultimately what had set Toothless off and left no more room for mercy in his eyes.
There was the Screaming Death, where their only way to take care of him was to fight him off and keep him as far away from Berk as possible until Hiccup realized that all the Screaming Death wanted was his mother. After which he reunited them and then had no further encounters until Hiccup's idea to use Screaming Death scales as armor to protect Toothless and Hookfang from dragonroot arrows.
Hiccup has always been consistently someone who doesn't like to unnecessarily get his hands dirty, but he will if the situation calls for it. Yet he's also someone who respects nature and the cruelty within it. Like when he first wanted to let the Catastrophic Quaken take Dark Deep for himself until Fishlegs figured out he was chasing the Gronckles away, not to claim territory, but because he feared the new faces. Another example is Vanaheim, he was super protective of that place and didn't want anyone to mess around with it, not even him and his Dragon Riders.
The only times he really interferes in dragon business is when other humans do or when their (also his) dragons were directly threatened. (Or that one time he found a baby dragon and took him in, though he didn't even know Torch was a baby)
Like when he trapped the Death Song and freed their dragons and one baby Thunderdrum because that was the deal he made with the adult Thunderdrum, but their dragons weren't the only ones trapped there in initial shots. Same thing when the Dragon Riders decided to stop the Cavern Crasher from attacking the Fireworm Queen's nest. Not only had she saved Hookfang's life, but she came to them for help. Yet when Singetails came to take the original Storehouse island, they put up a fight, but ultimately let the Singetails take it and they've been there for the rest of RttE. The island now belonging to them, even though Spitelout had clearly gotten there first.
Yet, there's no sign of this with Jörmungandr. No humans getting involved or signs that this was just nature taking its course, he got involved for who knows what reason. Was Jörmungandr messing with the homes of their dragons specifically? Or was it because he truly did push the balance in the Hidden World off-kilter so badly? And if he really was such a threat to the dragon world (idk, he seemed more like a gigantic bully to me, I need to rewatch those eps and I'm currently on season 4) than I don't see Hiccup's thought process behind locking this thing up for over a 1000 years as his solution for Jörmungandr when that is infinitely crueler than just killing him.
I mean, ending his existence vs 1000 years in a confined space with little food and water and no room to really move? The same brown walls to stare at with freedom just beyond a dragonproof grate for well over a thousand years? I think it's pretty clear which of the two the more merciful option is and it's not the one that has been scientifically proven to push humans to the brink in, like, a few hours. Let alone specifically 1 300 years.
I'm reminded of when he decided that the Skrill needed to be frozen back into the ice it came from in DoB versus his decision to ultimately let the Skrill go after it escaped, having watched the utter loss of fight in the Skrill's eyes as it just accepted it was going to be trapped all over again. Which Hiccup saw and was what made him decide against doing it again.
Maybe there's the possibility that the cage was only supposed to be a temporary solution and Hiccup died before he could come up with a permanent one? Dragon Club does find his prosthetic leg and tools in a space he clearly worked in (no bed, just a table, his leg and tools as far as I remember) and just why would he leave those things behind? Especially as a canonically disabled person who has lost his replacement limbs before, Hiccup wouldn't leave his prosthesis there unless... there was just no more use for it.
So yeah, I don't understand a lot of the things that Hiccup did in The Nine Realms, which makes this a difficult question to answer. (it's a shame, because at least one of the writers wrote some of my favorite RttE eps) I don't understand a lot of things Hiccup does from THW and onwards!
But if we take the idea that his solution with Jörmungandr was supposed to be a temporary one that he just could not solve before his own time ran out, then he'd probably feel guilty. Both for leaving Jörmungandr to be locked up for so long and that Tom and his friends had to deal with him in his stead.
