A neat story with an unreliable narrator.
1. It seemed predictable at first, just going on with its premise of a sadistic precognitive mother and her masochistic daughter. The mother guided her towards each and every bad experience she had. For instance, on the day when she was told to miss the bus, she met that guy because she was acting oddly at the bus stop due to the letter. In each case, the abusive people in her lives were all linked back toward what the letters advised her to do. And the daughter had to be masochistic to keep sheepily opening them a tthe assigned dates and following their advice despite the consequences.
2. Then as the story went on, it seemed more likely that the letters were not at all precognitive, and that she wrote them to herself after the terrible things happened. I don't konw why, perhaps to feel like she had a mother figure? Perhaps she had no good reason and is just insane. This would explain the similar handwriting, and also why the pages were blank (she hasn't written those letters yet).
3. At the end it seemed to twist again, and it seemed that there was no precognition, nor any intentional forging of the letters, but somehow this woman is trapped in a time loop where she is her own mother. She writes out new letters to her daughter-self each time, "predicting" her daughter-self's future, but it's really just a reiteration of her own past that will become her daughter's future and she is too insane to realize what she is doing.
In any case, I don't think the non-scented people were anything more than people. I think she'd been hurt by a few people in the beginning chapters, and rather than believe that human beings could be so cruel, she dehumanized them, giving them that scentless trait that seems so obvious to her but others will disbelieve. And as time goes on, she starts to assign this attribute to more and more others as she feels more distanced from humanity, and because they're inhuman to her, there is no boundary to killing them.
