One lesson of "A Poison Tree" is that if you hold onto your anger and nourish it, it will grow and hurt someone--in the case of this poem, it hurts an enemy, but in other cases, it can hurt the person who is angry, too. The poem is an extended metaphor in which anger is described as a tree.
In the poem, the narrator does not tell his foe that he is angry, so he says, "...my wrath did grow" (Blake line 4). As a tree needs sunshine and water to grow and thrive, the narrator nourishes his anger, saying, "And I sunned it with smiles/And with soft deceitful wiles" (lines 7-8) and he watered it with tears. In other words, he is dishonest with the person whom he is angry with, pretending to be friendly, while his anger grows and grows.
As the tree gets larger, it bears fruit, "an apple bright" (line 10), that his enemy steals into his garden and takes. Eating this apple, which is the fruit of the narrator's anger, kills the enemy, and the narrator sees "My foe outstretched beneath the tree" (line 16).
When we are angry and we say what is on our minds and let it go, it loses its power over us and others. When we hold onto our anger and obsess over it, it is toxic, hurting others, physically or mentally, and even hurting ourselves in body and spirit. The more we feed our anger, the larger and more harmful it becomes.