I actually really like this turn. The manga starts with that being the idea; that the girl is a serial killer and the only way they're ever going to stop her is if the gyaru who can see ghosts figures it all out by talking to the guy who was killed.
But slowly, as the story progresses, things just don't feel right. There so much history and pain, but it's not caused by Chiaki, but instead just suffuses all the relationships we see. She doesn't have any reason to do anything. She's odd, but nice. Rather than being a killer, she seems utterly wracked with guilt and prepared to kill herself at any time if given a reason. Meanwhile, Sahara keeps finding things out that don't make sense. His memories are wrong. We hear things about him that don't fit how we see him act as a ghost. He seems to be quick to deflect and find reasons why he didn't ever do anything wrong, but they feel more and more hollow.
Now here, at the denouement, we finally see it all come together. Chiaki isn't a horrible person, she's a broken one. She doesn't want to do anything but help, but the tragedy that happens around her forces her to become insular. She blames herself for everything that happened, but no one accepts it. Her empty eyed smile that we've been afraid of this whole time is her reaction to sudden strong negative emotions, conditioned as a child by her Grandmother and Sahara himself. It didn't come out of nowhere, either, she said that back in chapter 9. The reason she'll smile when something sad, or scary, or anything emotionally troubling occurs isn't because she's fuckin' loving it, it's cause her face moves independently of her thoughts as a defense.
Sahara has been running, this whole time, from the fact that his temper and impatience hurts people. It killed someone he saw as a little sister. He can't accept that part of him, but he won't change it. He instead denies it. It's always everyone else's fault. If everyone else would've just gotten their shit together, it'd be fine. He'll dodge responsibility like the matrix because he doesn't want it, even if he knows its the right thing to do. So everyone just swallows their feelings when he hurts them. Shoulders the guilt that should be his. Burdens themselves by saying "I just took it wrong," or "I shouldn't have been bothered," or "it's my fault Mii-tan died."
I'm pretty sure, actually, that both of them are alive. Just in a coma. That's why Ken needed to stick around and needed the scholarship to even go to school, Mii-tan is in the hospital there. Maybe Sahara will wake up when he finally accepts what he's been running from; that he abandoned a little girl and caused her to get hit by a car by leaving her alone. Maybe she'll wake up then, too.
It's been a mystery, but much less about crime and much more about characters, their history, and their interactions. It's all been really strong, and I'm glad that it's gone this route.