I think his reaction is honestly reasonable to some extent tho. He was basically a kid when he was thrown into an unfamiliar place when he was initially unloved in the first place. Then he meets someone and thinks he finally cares for someone and then doom she’s dead. The crutch that Asahi was given in the form of the water god and the people of the village who liked her kept her sane whereas his crutch was whipped away when he was already more unstable than her. He’s reached his breaking point and the self blame he’s placed on himself isn’t helping. The death god gave him a way out a way to get his sanity in the form of hino back in a world and combined with the fact that he knows the cruelty more than the kindness of humans is an easy deal. He’s envious that Asahi got kindness and blessings when he got his ripped away. Not justifying his actions just see where he’s coming from
Until he is hurt with the worst way possible:
He is losing himself, and if he loses his purpose, basically, he destroys everything, as that is the only goal he has now, there is nothing for him. Nothing. It will be so agonizingly painful and all that will be left is emptiness. (Not necessarily, just put him in a dark place for an indefinite amount of time, and he'll feel it. And then show him the sufferings of those he had hurt, and what thry had before. Show him how his life was, and what he had done, and where he is now.)
On the other hand, if he does remember his goal, the worst pain for him will be either the huge backlash of regret and the immediate collapse of everything, having such a bloody and twisted base for his castle of bubbles, or dissatisfaction and emptiness, upon which thr first "punishment", the one up there, would suffice.
Then, he'll be even more hurt. Then, in that instance of pain, hand him an olive branch of sorts... how he can possibly redeem himself. Up to him to take it or not, but let him not die.
Let him feel pain without that solace that he might end up seeking, let him not go insane despite the borderline madness that is chasing him like a plague, tearing him to shred over and over again...
...sorry. I hope he becomes free from that manipulator and either saves his sister or accepts her death or so and lives the rest of his life bearing his sins, atoning, and moving forward.
...just got a little dark there.
i really want to feel sorry for him but honestly he's become so wrapped up in himself that he doesn't care about anyone else- oh she's smiling so she must be happy. i had everything taken away but i'll just assume that asahi had a good life and resent her for looking happy.
it's...ridiculous. absolutely ridiculous and entirely irritating for someone to have that kind of self centered victim mentality that they use to justify whatever they do because "i was hurt first so it's okay to hurt others" just, no.