For they sleep not, except they have done mischief; and their sleep is taken away, unless they cause some to fall. Except they have done mischief - The night is their time for spoil and depredation. And they must gain some booty, before they go to rest. This I believe to be the meaning of the passage. I grant, also, that there may be some of so malevolent a disposition that they cannot be easy unless they can injure others, and are put to excessive pain when they perceive any man in prosperity, or receiving a kindness. The address in Virgil, to an illnatured shepherd is well known: -
Et cum vidisti puero donata, dolebas:
Et si non aliqua nocuisses, mortuus esses.
Eclog. 3: 14.
"When thou sawest the gifts given to the lad, thou wast distressed; and hadst thou not found some means of doing him a mischief, thou hadst died."