[Galin]

I would say that in Tolkien's world killing is the separation of body and spirit, rather than the reduction of spirit to nothing. Again I have to rely on quotations from the net (from Morgoth's Ring), but as far as Morgoth being physical...

'To gain domination over Arda, Morgoth had let most of his being pass into the physical constituents of the Earth -- hence all things that were born on Earth and lived on and by it, beasts or plants or incarnate spirits, were liable to be 'stained'. Morgoth at the time of the War of the Jewels had become permanently 'incarnate': for this reason he was afraid, and waged the war almost entirely by means of devices, or of subordinates and dominated creatures.'

And in connection with a possible return...

'(...) in seeking to absorb or rather to infiltrate himself throughout 'matter', what was then left of him was no longer powerful enough to reclothe itself. (It would now remain fixed in the desire to do so: there was no 'repentance' or possibility of it: Melkor had abandoned for ever all 'spiritual' ambitions, and existed almost solely as a desire to possess and dominate matter, and Arda in particular). At least it could not yet reclothe itself. We need not suppose that Manwe was deluded into supposing that this had been a war to end war, or even to end Melkor. Melkor was not Sauron. We speak of him being 'weakened, shrunken, reduced'; but this in comparison with the great Valar. He had been a being of immense potency and life. The Elves certainly held and taught that fear or 'spirits' may grow of their own life (independently of the body), even as they may be hurt and healed, be diminished and renewed.'*

'The dark spirit of Melkor's 'remainder' might be expected, therefore, eventually and after along ages to increase again, even (as some held) to draw back into itself some of its formerly dissipated power. It would do this (even though Sauron could not) because of its relative greatness. It did not repent, or turn finally away from its obsession, but retained still relics of wisdom, so that it could still seek its object indirectly, and not merely blindly. It would rest, seek to heal itself, distract itself by other thoughts and desires and devices -- but all simply to recover enough strength to return to the attack on the Valar, and to its old obsession. As it grew again it would become, as it were, a dark shadow, brooding on the confines of Arda, and yearning towards it.'


(marginal additon) 'If they do not sink below a cerain level. Since no fea can be annihilated, reduced to zero or not-existing, it is no[t] clear what is meant. Thus Sauron was said to have fallen below the point of ever recovering, though he had previously recovered. What is probably meant is that a 'wicked' spirit becomes fixed in a certain desire or ambition, and if it connot repent then this desire becomes virtually its whole being. But the desire may be wholly beyond the weakness it has fallen to, and it will then be unable to withdraw its attention from the unobtainable desire, even to attend to itself. It will then remain for ever in impotent desire or memory of desire.'

I'm pretty sure this is all from Myths Transformed, Morgoth's Ring. Possibly text VII?