Blake's "The Poison Tree" from his Songs of Experience is about how nursing anger and hate corrodes our souls. The poem begins by stating that:
I was angry with my friend/I told my wrath/My wrath did end.
In other words, its best to make a "clean breast" of our feelings.
However, most of the poem shows the narrator nursing a bitter grudge against an enemy. It describes how this hate, unconfessed, grows like a plant and takes over the narrator's soul. He waters his grudge with his "tears" and suns his hate with his "smiles" of deception towards his enemy, pretending to like him.
The hate bears fruit, an apple. At this point, we should think of the Garden of Eden and the apple that the hate-filled Satan offered Adam and Eve.
The narrator has become Satan, his soul to turned to evil. He tempts his enemy with the "apple bright" and like Eve, the enemy falls for it. As the initial apple brought death to Adam and Eve (eventually), so the eating of this apple kills the narrator's foe.