Melkor was one of the most powerful of the Ainur. But why did he turn into such a shithead and how did he become known as Morgoth? Tl;dr: Boredom and loneliness. No, really. Oh, and also a good heaping measure of wanting the power of creation for himself, that too.
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He may have been super skilled, but Fëanor might just have been the biggest dick in the history of Middle Earth. Allow me to make my case.