If I had to guess, Sparkle switched sides upon seeing a vision of the Avatar annihilating her kind, possibly from her counterpart's perspective, though possibly not the context behind his actions. But as to why, I can only speculate. It has to be related as to why her spell chose David.

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It's a good guess, and you've got some bits close, but no. I'm glad to be writing something that provokes so much speculation though :-)