He has outsoar'd the shadow of our night; Envy and calumny and hate and pain, And that unrest which men miscall delight, Can touch him not and torture not again; From the contagion of the world's slow stain He is secure, and now can never mourn A heart grown cold, a head grown gray in vain; Nor, when the spirit's self has ceas'd to burn, With sparkless ashes load an unlamented urn.
Adonais-Keats has flown so high, the speaker says, that human problems like "envy," "calumny" (slander), "hate," "pain," and "unrest" cannot torture him. We guess that's a bright side.
Shelley is again implying (or maybe just hoping) that criticism can no longer bother Keats.
The corruption of the world is like a stain or a contagious virus, the speaker says, and it can no longer spread to those who are dead, and that's a good thing.
Adonais-Keats also doesn't have to worry about aging; his heart won't grow cold and his hair won't turn grey.
He gets to be young forever, remembered (and lamented) as a great poet who died young.
This is another reason not to mourn him, says the speaker.