Can you list the moral values ("we must," "we should") in the poem "A Poison Tree" by William Blake?

In terms of the categories "we must" and "we should," the poem says we should talk to a person when he or she makes us angry. This way we can work out our problems and get over our anger. As for a "must," we must not hold on to anger or it will grown into a hateful and destructive force. The moral values the poem upholds are forgiveness and honesty (telling a person when they make you angry).

The poem illustrates the "should" when the narrator makes up with a friend who has wronged him: "my wrath [anger] did end."

The poem illustrates the "must not" with the narrator's example of nursing his anger against an enemy who hurt him. He compares the anger to a plant that he "water'd" with his "fears" and "tears," and "sunn'd" with smiles and dishonesty ("wiles") by pretending to like his enemy. This anger grows into a poisoned apple that kills.

The poem says that our inward, bottled-up anger, fear, and dishonesty will eventually become outwardly destructive if we do not control it, so to build a less destructive (poisoned) world we must cultivate forgiveness.