What happened when the narrator of "A Poison Tree" did not express his anger?

In "A Poison Tree," the narrator says that his anger grew.  And he uses the growth of the tree as an extended metaphor for his anger, his wrath, viewing the anger like a tree that must be  given sunshine and water, so it will grow.  He waters it with "tears" (line 6) and suns it "with smiles" (line 7).  And the tree, or his anger, rather, grows and thrives, "Till it bore an apple bright (line 10). This apple, the fruit of his anger, attracts the narrator's foe, who eats it, only to be killed by the toxicity of the narrator's anger.  When we hold onto our anger, it is toxic, to our foes, certainly, but also to ourselves.  While there is no doubt some righteous anger to be felt for various reasons, it is keeping it to ourselves and holding onto it that causes the most harm.