The theme of William Blake's "The Poison Tree" looks deceptively simple (anger), but it's not.  Rather, the theme lies in how suppressing one's anger can actually make it grow more than it was before.  Blake presents a Old Testament-esque Christian allegory, similar to the Garden of Eden story, to indirectly reveal his themes of forbearance, self-restraint, and moderation.

The original title of the poem in his anthology Songs of Experience was "Christian Forbearance."  Speaking from experience himself, Blake says that secretly hiding angry feelings from others will only let them fester until they destroy both parties.   So says Enotes:

The principal theme of "A Poison Tree" is not anger itself but how the suppression of anger leads to the cultivation of anger. Burying anger rather than exposing it and acknowledging it, according to "A Poison Tree," turns anger into a seed that will germinate. Through the cultivation of that seed, which is nourished by the energy of the angry person, wrath grows into a mighty and destructive force.

William Blake's allegorical poem, "A Poison Tree," is concerned with the subject of anger. The speaker of the poem reveals to his friend that he is angry, and the anger dissipates. But when the speaker hides his anger from his enemy, the anger grows, much like a tree. The fact that the tree is "poison" tells us that, by ignoring or suppressing anger, we are also poisoned. The apple that appears on the tree of anger symbolizes that poisonous effect. The final line is ominous; the speaker's delight at the "foe outstretched beneath the tree" offers a warning about what happens when we suppress our anger. If we ignore or deny our feelings, we will become wicked, bitter, and even vengeful.