Thank you for the work, Bobby!
So.
While there is no real rule about stopping midway through the 100 stories, the spirit Yuuma managed to summon into Hina's body made it clear that she (it?) would keep hounding him until he tells the 100th story. Her hounding had cost the policeman his arm, Yuuma's dad his life, and Yuuma's stepmom her brain; the only reason she had to keep Yuuma alive would be to tell the 100th story, so one way or another, he would have to tell the story.
He's messing with a force beyond his comprehension, so his only way out—to nip the stories in the bud before anyone leaves Hell, so to speak—is by ending himself.
He could've chosen to rob his ability to speak, but since the spirit could still make Hina speak while her head was turned around whole (can you imagine the anatomical disaster behind that?) and even make Umakin claim a life, I think he would've suffered way more.
Also, on the brighter side of things, his suicide may finally bring the abuses he'd suffered into heavy scrutiny. His dad and stepmom won't get to live it down quietly for the rest of their miserable afterlives, even if stepmom recovers.
Hina also showed that she has the balls to go over the ritual again, knowing that it has real repercussions, so she bites back. If the police need a lead, she'll definitely provide.
Yuuma's death will grant him justice and it finally reunited him with his mother.
The only remaining loose end here is really Hina, and that's the exact loose end the mangaka picked up for the cycle. Beautifully done.