The gods are just, and of our pleasant vices
Make instruments to plague us:
The dark and vicious place where thee he got
Cost him his eyes.
Th' hast spoken right, 'tis true.
The wheel is come full circle, I am here.
"Full circle" is Edmund's coinage, and he employs a now-rare meaning of "full": "complete." The wheel of fortune has completed its circuit, and Edmund's own villainous acts have returned to haunt him. In essence, Edmund's use of "full circle" is close to ours—we too mean that someone's actions have passed through phases only to return to their starting point; but we no longer ascribe the outcome to the inevitable workings of Fate.
Edmund is the bastard son of Edgar's father, the late Duke of Gloucester. The duke has been blinded by Lear's enemies, but Edgar blames this on Gloucester himself. His adulterous lust—the "pleasant vices" that resulted in Edmund's conception—"cost him his eyes." This highly moralistic view of events relies on a notion of divine justice hardly operative in this play. Nonetheless, Edmund must agree; he has just been mortally wounded by his half-brother, and he interprets this as the result of his betrayal of Edgar. The villain Edmund had turned the duke against his legitimate son Edgar; the duke's lust produced Edmund, and therefore Edmund's villainy; Edmund's villainy, now come full circle, produces Edgar's successful revenge.