Can the Valar Sentence People to Death?

[Inziladun]

Originally Posted by Galin

As an aside: I would say Morgoth was killed; he incarnated himself and had become wedded to his body it seems, and was ultimately judged and executed.

I don't have access to Morgoth's Ring, but the Silm text says:

But Morgoth himself the Valar thrust through the Door of Night beyond the Walls of the World.....and a guard is set forever on those walls, and Eärendil keeps watch upon the ramparts of the sky.

If Morgoth was "dead", why the need to "guard" and "keep watch"?

I don't know which text was the preferred one from the Professor's point of view, but doesn't the Second Prophesy of Mandos refer to Melkor returning at the end as well?