Theory:

The summary of "A Poison Tree" how the suppression of anger drives to the culture of anger. Conquering anger rather than showing it and accepting it, according to "A Poison Tree," turns anger into a seed that will grow.
 
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William Blake’s “A Poison Tree” primarily uses two symbols (an apple and a tree) to relate its meaning. The tree depicts the growing anger in the speaker’s heart against his opponent, and the apple symbolizes the “fruit” of that anger, we should pardon our friends as well as our enemies and that we should not hold hatred. Once the poet softens down from his anger, he will realize what he has done and will be deeply hurt by it.