Live thou, whose infamy is not thy fame! Live! fear no heavier chastisement from me, Thou noteless blot on a remember'd name! But be thyself, and know thyself to be! And ever at thy season be thou free To spill the venom when thy fangs o'erflow; Remorse and Self-contempt shall cling to thee; Hot Shame shall burn upon thy secret brow, And like a beaten hound tremble thou shalt—as now.
The speaker condemns the critic to live a long life. Um… say what?
He wants the critic to live with what he's done, even if nobody else knows about it. He says the critic's "infamy" is not his fame, meaning that he's not famous for his criticism.
The critic is just a "noteless blot on a remember'd name." In other words, the critic is just a stain, unworthy of comment, on famous name—sick burn.
Again we get a simile, comparing the critic to a snake, saying that the criticism is like "venom." The speaker knows that the critic will continue to spread this poison.
But even so, the critic knows, and should be filled with, "Remorse," "Self-contempt," and "Hot Shame" forever, burning on his "secret brow" (forehead).
The speaker hopes that the critic will tremble fearfully, like a dog who has been beaten.