Since it's all nearly over...i'm gonna spoil you. Don't read on if you don't want to be spoiled! Although it's possible what i'm telling you isn't very obvious in the story. Heh.
The souls of the children were used to bargain for the knowledge of what souls to take and why, and how.
Sam's soul gave her power - Sam and Dean's together would be like a nuclear bomb. Rafe...well... What does an angel have to give, that a fallen angel doesn't have anymore?
Divinity. He's the cloak that will cover her so she can slip in the back way, as it were.
The rune, glyph, whatever, locked up her power. Even fallen angels have a lot of it. And if your power - what makes you not-human - is locked down and taken away... She made herself a 'human' shape to move among the humans. Losing her power meant she was locked into that shape for all time. And that shape was too badly damaged to live.
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The souls of the children were used to bargain for the knowledge of what souls to take and why, and how.
Sam's soul gave her power - Sam and Dean's together would be like a nuclear bomb. Rafe...well... What does an angel have to give, that a fallen angel doesn't have anymore?
Divinity. He's the cloak that will cover her so she can slip in the back way, as it were.
The rune, glyph, whatever, locked up her power. Even fallen angels have a lot of it. And if your power - what makes you not-human - is locked down and taken away... She made herself a 'human' shape to move among the humans. Losing her power meant she was locked into that shape for all time. And that shape was too badly damaged to live.