I imagined that the "witch" (I use that term loosely, she is never referred to as a witch until she goes against the family's wishes) pulled together what parts of a soul were in the womb, and bound them together with Talia and Amond's affections. Over time this binding fused together the soul into a mostly viable person, but when the spell was broken there were large gaps left. Holes cause a combination of emotional and physical pain where an aspect of her (such as her capacity for empathy) once was while the parts there are (such as anger) attempt to fill what is missing. It's similar to those who feel pain in phantom limbs, which is caused by the brain overtaking the parts used to control the missing limb and applying them to different tasks.
These holes are not a terribly uncommon occurrence in people but the holes most have are small and get repaired with time or stoppered with something such as drugs or a new puppy. Unrepaired souls make a fabulous foothold for evil influences which is why there is usually some trauma associated with those who walk that path. What makes that route doubly tempting is that when someone is killed their soul momentarily passes through that of the killer and fills in those gaps for a few seconds. Beating a person within inches of their life brings them within touching distance and has a similar though less dramatic effect, as does intimate sex (though the conditions for that are difficult to meet as it requires nothing short of true love).
In effect, Snap's soul is filled with many large gaps and cracks making it a breeding ground for the essence of the nether realms. She's caught between the person she knew she was, and the person she feels she is. The monks taught her to manage the constant pain, the consuming rage, and the breeding evil, but none of it ever goes away. If she ever lets her guard down at least one of these will overtake her, and no one is sure what happens after that.
I want this to be the focus of a save-the-town quest. Everyone is starving, quickly! Find and lug the heavy stone object through the dungeon, across the rickety canyon bridge (WHUMP!), over the lake in the leaky boat (*gurgle*), up the hill, up the hill again, (Damn you Sisyphus!) et cetera. It makes me snigger with evil DM delight.