What is the theme in A Poison Tree by William Blake?

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.