An Elf with shapeshifting ability? Why?

[Lord Melkor]

Of course, the question is whether 'killing' Morgoth's physical form would have any longterm benefit. Though Morgoth was thoroughly incarnate by the time Beren and Luthien came to Angband to steal the Silmaril, he had infused Arda with so much of his power that the whole of Arda basically served as his very own One Ring. As such Morgoth could never be permanently defeated until the end of the world when Arda Marred would be remade and slaying his physical form would have no long term effects, except giving Elves and Men a brief respite while Morgoth fashioned himself a new shape.