why did dantes say this "And now," said the unknown, "farewell kindness, humanity, and gratitude! Farewell to all the feelings that expand the heart! I have been heaven's substitute to recompense the good -- now the god of vengeance yields to me his powe

Dantes swears this in prison. He knows retribution and vengeance exacts a price which is why he says farewell to security and happiness. Dantes wants to punish his wrongdoers even more than simple death. He wants them to suffer the loss of kindness, humanity, gratitude....just like he did.