Satan's despair is absolute because Satan, as pure spirit, is pure consciousness, and for Satan (and all men in his predicament) every increase in consciousness is an increase in despair.

With every increase in the degree of consciousness, and in proportion to that increase, the intensity of despair increases: the more consciousness the more intense the despair.

With every increase in the degree of consciousness, and in proportion to that increase, the intensity of despair increases: the more consciousness the more intense the despair

All his court were cast down in slumber, and all the fires faded and were quenched; but the Silmarils in the crown on Morgoth's head blazed forth suddenly with a radiance of white flame; and the burden of that crown and of the jewels bowed down his head, as though the world were set upon it, laden with a weight of care, of fear, and of desire, that even the will of Morgoth could not support. Then Lúthien catching up her winged robe sprang into the air, and her voice came dropping down like rain into pools, profound and dark. She cast her cloak before his eyes, and set upon him a dream, dark as the outer Void where once he walked alone.

In the vacuum created by fear and ignorance and hunger and want, it's evil, not good, that rushes to fill the void.

How shall a man judge what to do in such times?''As he ever has judged,' said Aragorn. 'Good and ill have not changed since yesteryear...It is a man's part to discern them, as much in th Golden Wood as in his own house.