In “A Poison Tree,” the speaker is most certainly vengeful – he speaks of nurturing anger, not of expressing it but of letting it fester within him, allowing it to grow by giving to it the elements of his sinister plotting – “…smiles/and soft, deceitful wiles.” He is thus appearing to a “foe” as if he were a friend, planning all the time a way to get back at this person for some undefined fault.
The contrast should be mentioned between how the speaker reacts to anger at a friend versus anger at a foe – in the first verse he voices his ire at a friend, and the ire therefore dissipates. However, because he doesn’t have the same relationship with his foe as with his friend, he says nothing about his anger and instead allows the nature of their relationship to kick-start this evolution of anger into revenge.
In the third stanza, Blake writes, “And it grew both day and night,/Til it bore an apple bright.” This apple is the physical representation of the nefarious acts born from wrath, and which poisons not only the victim of the revenge, but the tree from which it springs forth – in this case, the speaker himself. This is illustrated definitively at the end of the poem, when the speaker is “glad” to find his “foe outstretched beneath the tree.” If we interpret this to mean that his foe has taken the poisoned apple, has been deceived by the speaker’s cunning vengeful plot, and has been killed as a result, we can then assume that the speaker’s character has indeed been poisoned, for he rejoices in the death of another man -- moreover, a death of the speaker's own invention.
The poem therefore notes the terrible transformation that stems from burying one’s emotions rather than expressing them – by allowing one’s anger to turn to wrath, by nurturing it with guile and negativity, one will not only poison one’s relationships but oneself as well.
As with most of Blake’s poems, there are many layers of symbolism within “A Poison Tree” – for more, and for the religious and historical context of this poem, check out the eNotes analysis page.