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.