There is no Good End
The impossibility of conflict ending favorably for everyone
Kissshot was searching for a place to die and was on the brink of death after having her limbs stolen by a group of three vampire hunters, Episode, Dramaturgy, and Guillotine Cutter, when Araragi saved her by giving his blood and becoming a vampire himself. Skip to the end, after regaining all of her powers as a legendary vampire, Kissshot wants to meet her end by having Araragi suck her blood so he can return to being a human and she can finally die. To the dismay of Kissshot, Araragi refuses to drink her blood because of how he values her life. Instead, Oshino, a specialist in aberrations, arrives and tells Araragi the three choices he has and how they play out, but none of them are a happy ending for everyone.
In one case, Araragi kills Kissshot, he becomes a human again, no one else will be eaten by Kissshot, and the vampire hunters will be able to live knowing that Kissshot finally died, however, Araragi will live with the guilt of killing the being that gave him a second chance at life.
In the other, Araragi let Kissshot live, more people were going to die because of Kissshot, she would be forced to live against her will, and the vampire hunters would have to deal with the fact that they failed to destroy Kissshot before she could return to being a full-powered legendary vampire.
The third option Oshino suggested, and the one that would make everyone miserable, would be for Araragi to take almost all of Kissshot’s blood, leaving her as a powerless husk of her former self, forcing her to live as a lesser being. Araragi would never return to truly being a human and the vampire hunters would know that Kissshot is still alive and, though dormant, could one day regain her power and return to her true nature as a monster. Unable to deal with killing Kissshot or allowing her to kill anyone else, Araragi chooses the third option, the least desirable of the three for everyone and the one that spreads misery out among all parties equally.
Prior to this, Araragi stalled the process of turning back into a human (which he didn’t know the details of at the time) and went to the store to grab some food. When he returned, he found Kissshot eating the body of Guillotine Cutter, one of the vampire hunters who almost succeeded, a fact about vampires that he had kept out of his mind the entire time. Guillotine Cutter went along with Oshino’s plan because he was given the promise that Kissshot would die turning Araragi back into a human, but with Kissshot still alive because of Araragi’s stalling he decided to take matters into his own hands and died in the process. Death is the one thing Araragi wanted to prevent most of all, more than he wanted to return to being a human, and he was unable to ensure his own happy ending, regardless of his later actions, because of this.
The lesson to be learned here is that however a resolution may be reached, where there is conflict there are always losers, and sometimes there aren't even any winners. Though there may be many paths to take to a resolution, there is always a give and a take, and it is never the case that everyone ends up happy with the results.