I wouldn't say ending spoils the mood but makes presumptions most have to be proven false, because she was avoiding the true reason and lying to others and possibly even to herself.
You in a sense could stop reading at page 28 to keep it within the drama/mystery category with possibly happy/hopeful undertones. I would even suggest that if you don't have a soft spot or just don't like tragedies and you like the story up to that point, you should.
Not every story has to have happy ending, this is more of the type that turns this story from your average mystery/drama to a tragedy due to reveal, since she never can have happiness, either she will be for ever the hungry wolf or turn on her friend. As a continuation of the story, she will always* have a tragic outcome.
One silver lining that you could see in this story as well (tragedies combined with catharsis and silver lining have a soft spot on my mind) is that maybe, just maybe, Kagerou will one day tell the truth and Wakasagihime will sacrifice part of herself to sate the hunger and maybe let Kagerou hopefully get past the memory/hurdle, even though it might not be enough. And based on the grasp on the personality of Kagerou within this story, it makes me think that she will stay a hungry wolf rather than lose Wakasagihime or even worse, hurt/kill her.
Yeah. Since it's left open, one could just imagine that Kagerou works through her temptation. She doesn't have to give in to it. Maybe she resolves to run away and avoid it altogether. I'd still rather have the happy ending though. Oh well.