Prospero certainly did not always feel this way. In Act 1 he was so adamantly telling Miranda what evil bastards Alonso and Antonio are and it is said that, "He that dies, pays all debts." Later on, Prospero comes to the realization that he values his own peace of mind over vengence. He forgives because he has evolved as a character, releasing himself of his magic, and going back to his true weak self. Ironically, letting go of his power sets him at ease, as if he was filled with the plague of honesty. I also think if you cannot fully comprehend the book, you should not be looking into its paralells to Shakespeare's writing career just yet.
