What is the message of the poem "A Poison Tree"?

One difficulty with untangling the "message" of any of Blake's work is that Blake himself had his own, extremely complicated personal religion and most of the elements of his poetry refer to parts of this religion. Since these beliefs are ones derived from voices he (but no one else) heard, and the accounts of the pantheons in this religion vary from poem to poem, untangling the actual meaning or point of his poems can be...

One difficulty with untangling the "message" of any of Blake's work is that Blake himself had his own, extremely complicated personal religion and most of the elements of his poetry refer to parts of this religion. Since these beliefs are ones derived from voices he (but no one else) heard, and the accounts of the pantheons in this religion vary from poem to poem, untangling the actual meaning or point of his poems can be difficult; there is a substantial amount of literary criticism devoted to this task.

First, whenever we encounter a single apple or fruit in a garden, we are seeing a reference to the Garden of Eden and the fruit of the tree of knowledge. In the Bible, eating this was part of Original Sin which got Adam and Eve expelled from the Garden and caused all sort of ensuing problems for the human race. In Blake, Eden was not a singular event, but a wellspring of creativity and a mental state to which he could return. 

The anger he expresses to his friend enables him to be at peace with himself. The anger he keeps towards his foe is not expressed, and leads him towards hypocrisy and inauthenticity, which are part of human fallenness. The fruit of this anger, though, kills his enemy, and thus rage and rebellion also serve a positive purpose; as Blake admired Satan in Milton's Paradise Lost, inter alia, one should normally assume that his treatment of Christian material inverts many of the values that Christians would impute to various Biblical narratives.

Since God created the tree of which Adam and Eve ate to get expelled from Eden, the parallel we should draw is that God is a foe of humans and created the tree out of concealed anger.