How the mighty do fall!

[Inziladun]

Originally Posted by Zigûr

It depends what you mean by "killed" though, doesn't it? Morgoth was literally killed when he was executed by Mandos at the end of the First Age, as in his body was damaged so much that it stopped working and his fëa could no longer resided within it.

I take "killed" to mean the state for any sentient creature in Arda whose physical form is separated from the fea.
In terms of Ainu, that should only apply when the forms are 'real', such as the Istari, who were ensconced in actual bodies which they could not at will discard, and Sauron, who had built himself a form that was tied to the world.