Montresor, the main character in "The Cask of Amontillado" emodies the theme of how letting old grievances rankle and fester inside of you produces a very bitter and poisonous fruit indeed. Look at the parallel comparisons. Montresor opens his tale by mentioning the "thousand injuries" and "insult" that his friend Fortunado had given him. But instead of going up to Fortunado and talking to him about these grievances, he keeps them inside and dwells on them eternally, until his anger has grows and grows, getting stronger and more intense. William Blake lets us know that if you have a grievance, "I told my wrath, my wrath did end." Talk it out, confront your friend, and it will be over. Montresor never does this. Instead, he "told it not" and his "wrath did grow." Eventually, his anger "bore an apple bright" in the form of sweet revenge. Apples are sweet, and revenge was sweet to Montresor. He took delight in planning his revenge, and enjoyed executing that revenge. As Fortunado struggled vocally in the wall, Montresor even stopped his work to sit down and rest a bit to "hearken to it with the more satisfaction." He was truly, enjoying the sweet "apple" of revenge.
Montresor models exactly Blake's assertion that letting old insults fester and fester and fester only makes the anger grow and get stronger, taking a life of its own. And, that life might produce what seems to be a sweet fruit; however, note the title of Blake's poem--"A Poison Tree." The apple is sweet, but poisonous. Montresor's revenge was sweet to him, but murder was the price. A death occurred; a man's life was cruelly sacrificed on the alter of Montresor's sweet revenge. I hope that those thoughts helped a bit; good luck!