Sils and Morgoth

[Voronwe]

Being killed, in this case, refers to the destruction of the body, or physical form. It has nothing to do with the destruction of the spirit. (in fact, I think Tolkien once said that he believed the spirits of created beings were completely indestructible, though I might be wrong on this)

The Valar could all, in the beginning, take on any physical form at will. Melkor lost this ability because he let so much of his power pass into corrupting the matter of Arda. He became stuck in one body, which is why being killed (assuming he was) was so damaging for him, as his spirit was left houseless and significantly weakened.

I tend to agree that Tolkien's latest writings in HoME are a better guide to his intentions than the published Silmarillion. However, I think it really comes down to a matter of choice whether one accepts the published Silmarillion as fact, or delves into HoME and all the 'Canon' issues it entails. Arguments over whether Morgoth was killed and similar issues won't really get anywhere, if the two sides are arguing from different premises. At best one can merely claim that Tolkien's writings in HoME were his last opinion on the subject, which he might have changed again later.

-Voronwë