How about some sort of reverse hag:
The changeling got a taste of the fae side and decided, that he does not want to go fae, so he decided to be human. However, a curse from his fae relatives keeps him from being able to choose to be human and draws him slowly towards being fae. To prevent this, he is trying to collect humanity from others, specifically those who are considered the best of humanity (local heroes, social workers, etc.), but also those who are the "best" at enjoying human life (hope you know what I mean. Those who live their life large, that type of people.). Of course, this is not really working too well, because he is both getting more human by collecting that humanity, but by using his powers to do so, he is also becoming more fae. It is not hard to imagine, that this is driving him insane.
In powers he would have a feeding dependency attached to his fae powers that would be satisfied by taking the "essence" of someonce life, basically a ritual killing (pretty much like a wcv does, only instead of emotions he takes the "soul" (I know, Bob would rattle on his shelf now, but for this purpose, the soul is the essence of someone, who they really are.).
The interesting part is, that once he takes someone's soul, he also takes on part of their personality, so this should probably be reflected in their behavior (maybe via the mimic abilities power). For example, the first murders do not show any remorse, but then an aspect of someone makes him change his MO to take care of his victims, after they die, he makes sure, the bodies lie "comfortable", eyes are closed, etc..
Soulgazing this changeling would probably be like walking into a psychiatric clinic with _really_ insane persons inside. I would describe this as a long corridor, very badly lit, on both sides are reinforced doors with a small window, and inside each room is one of the changelings victims. At the back of the corridor there is a single open door that leads into a pretty large room. Inside that room, there is a line on the floor, that divides the room in 2 sections. In the smaller part, even smaller than those rooms with the imprisoned souls, is a humanoid, strangely twisted, like it has been changed back and forth between two shapes and is now caught in between those forms. It always moves towards the line, but then throws itself back at the wall, before it can cross the line.
To sum up the powers:
- Glamours [-2] because using it makes stalking his pray a lot easier, AND it triggers the Fae Curse.
- Mimic Abilities [-3 at least, maybe more] This might not trigger the Fae Curse, because it is part of his rather unique situation instead of his fae powers, but they might come from them too. To increase the impact of the Fae Curse, this should trigger it, too.
- Fae Curse [-1]
- The Fae Curse draws the changeling towards the fae side and compels him to give into his fae nature. Due to this, he gains a +1 on any roll involving an ability that is part of his fae heritage. (like "Demonic Co-Pilot", use the Discipline rolls and mental stress as described in the book. Consequences that result from this can only be removed by feeding on a human soul. These consequences can be compelled to trigger a "feeding frenzy", that makes the changeling chose a new victim.
- Soul Eater [-1]
- The changeling is able to eat the soul of it's victims and make it part of his own. For every soul he devours he can reduce all his mental consequences from the fae curse by 1 step (mild is removed, medium becomes mild, severe becomes medium). There is no other way to remove those consequences.
- Thus far we are at -7, if you don't increase Mimic Ability (you might have to experiment with how many points you actually need there). You might want to take inhuman Strength/Speed/Toughness/Recovery, to reflect his more-than-human physique. Great for triggering the Fae Curse, as well.
