Spoiler!
Thesilid wanted to go there to destroy it, because that's how the heroes managed to imprison him in the dark dungeon. To prevent him from regressing or using any of his transcedental powers to free himself, they forced him to drink the water from this well for hundreds of years, effectively torturing him in captivity rather than let him regress and risk him killing innocents again. He wants it gone to prevent that traumatic experience from ever happening this time. He doesn't have a death wish, it's the opposite — around Ailette, the world is different, and he is able to find hope to live on.
Thesilid realising that even Ailette's obvious love for treasure is nothing in the face of her affection for him has him so shocked lmfao I love them sm
Psst. You should check these out (they're fuc**** awesome): https://mangabuddy.com/manga-list/210385