That is a complex question and one fraught with peril. Its like why would God let Satan loose on the earth to cause evil?
As you said, it was part of the One's grand Purpose, of which I am ignorant. In my mind the whole thing is a metaphor of Good triumphing over Evil. Evil leads to ugliness, decay, and death...but the pure heart even in the darkest fire is purified like gold and shines brightly.
Also it shows that pride leads to folly and everyone who would be a Dark Lord (Morgoth, Sauron, Feanor, Ar-Pharazon, Saruman...et al) even in the smallest way, falls into ruin and only achieves Eru's greater End.
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only that Part from which he came
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