How the mighty do fall!

[Zigûr]

Originally Posted by Belegorn

You're on the verge of thinking like Melkor about destroying matter being the end all be all. Yea his body was destroyed after he was decapitated and that ended him as an incarnate being, but he lives on.

I just came back upon this thread and noticed this. I am aware that Melkor "still lives" after the War of Wrath and that his fëa cannot be destroyed, but that's true of all beings in Eä - being killed just means that the hröa can no longer house the fëa. To my mind this corresponds with what Professor Tolkien says in the essay found in Morgoth's Ring:

Morgoth was thus actually made captive in physical form, and in that form taken as a mere criminal to Aman and delivered to Namo Mandos as judge and executioner. He was judged, and eventually taken out of the Blessed Realm and executed: that is killed like one of the Incarnates. It was then made plain (though it must have been understood beforehand by Manwe and Namo) that, though he had 'disseminated' his power (his evil and possessive and rebellious will) far and wide into the matter of Arda, he had lost direct control of this, and all that 'he', as a surviving remnant of integral being, retained as 'himself' and under control was the terribly shrunken and reduced spirit that inhabited his self-imposed (but now beloved) body. When that body was destroyed he was weak and utterly 'houseless', and for that time at a loss and 'unanchored' as it were.

"killed like one of the Incarnates" suggests to me that "killed" in this context specifically means "having the hröa destroyed such that the fëa can no longer reside in it." Just because Melkor's fëa still exists doesn't mean he wasn't "killed". Elves can be killed, but their fëa persist and can be reborn. Men can be killed and their fëa go to Mandos. It's less common for an Ainu to be killed, but if they become bound to a single body, as Morgoth did, they can. It doesn't mean they can't recover from that, because we know Melkor could, but we wouldn't say Elves can't die or be killed just because they can be reincarnated, would we?

As Inzil points out above, Gandalf and Saruman were both also killed, despite being maiar: the hröa to which the fëa was bound was destroyed. Otherwise we might as well say that Wizards, Elves, Morgoth and practically anyone else besides Men and Dwarves is only "disembodied" or "temporarily inconvenienced" rather than "killed".

Sauron is also described as being "slain" in letter 131, in reference to a time when he appears to have been bound to a single body, at the end of the Second Age: "Gilgalad and Elendil are slain in the act of slaying Sauron". I note, however, that in the Appendices to The Lord of the Rings Professor Tolkien uses the more ambiguous term "overthrown", perhaps to simultaneously indicate the breaking of his power as well as his personal (temporary) demise and to avoid confusing readers who might be wondering how he seemingly "came back to life" given that it's not very clear in The Lord of the Rings itself as to what manner of being Sauron is.