Remember how Snow White used dust from a bad fairy to turn trolls to bugs in order to save Prince Charming? She was saving it to transform the EQ and squash her like a bug. I suppose fairy dust can be either good or evil, depending on either the source or the one using it.
In the case of the “pure evil” remark, perhaps Rumple thinks the consequences of magic are often worse than the desired result.
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"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy