What is the message of the poem "A Poison Tree"?

The message of the poem is that if we hold anger within and nurture it, it is poisonous and can harm others.  In the first verse, the narrator sets the stage for this message by stating that when he is angry with someone and tells the person, his anger ceases.  But when he keeps his anger to himself, anger with "a foe" (line 3), his anger grows.  While the narrator makes a distinction in the poem between friend and foe, I think that this distinction is not all that important in terms of human emotion, since anger held in can just as easily be toxic to friend and foe. 

As the poem goes on, the narrator uses the metaphor of a tree to show what happens to the seed that begins as anger. He tends to the tree with fears, tears, false smiles, and "deceitful wiles" (line 8). This is the narrator feeding his anger by holding onto it, rather than simply letting it go. Ultimately, his anger bears fruit, the apple on the tree. When his foe sneaks into the narrator's garden and eats the apple, he dies from its poison. Thus, the narrator's anger has killed his foe.