Blodrin

Blodrin was a treacherous Elf whose story was told in the Lays of Beleriand.

Background
He was an elf, the son of Bor. As a child the hostile bearded dwarves dragged him to their deep mansions, and in Nogrod he was nurtured and tought to spite his blood and birth. To hate both elves and Men. He developed a personality similar to the wicked dwarves] he grew up among.

Later in his life,  he joined Turin's outlaws, but like the dwarves he lusted for treasure (his greed for gold was great as great as the dwarves he had been raised within, and as great as the hell-dragons), but he hated Hurin's offspring and the bowman Beleg. He  fled their fellowship and forest dwellings and allied with Orcs. He bargained with them for gold, and thus he betrayed his fellowship to the Orcs in the forest hideouts. The traitor and the orcs killed many men through treachery. Turin was captured. Beleg was buried in a pile of bodies, but escaped death. But in the end Blodin was killed by a poisoned arrow to the throat held fastly to the tree behind him.

The sinister effect of Elvish association with the wicked dwarves had on Blodrin followed after that as it had done to Ufedhin.

Behind the scenes
The story of Turin's traitor went throughs several revisions, ultimately apparently became the basis for the emergence of Mim as the traitor of the outlaws who had allied with the orcs, and betrayed their location.