I would think that the main theme of this excellent poem, which can be related to your question, can be clearly seen in the first stanza, which states:
I was angry with my friend:
I told my wrath, my wrath did end.
I was angry with my foe:
I told it not, my wrath did grow.
There is a clear division here in terms of how we deal with anger and the consequences of anger that is not expressed and dealt with. In the first instance, the speaker, feeling angry towards their friend, yet clearly still considering that individiual to be a "friend," tells their friend about their anger, resulting in the end of that destructive emotion. However, the other instance narrates how the same speaker felt anger towards his "foe" but did not tell him about it, and thus the anger grows as a result, ending in the death of his foe. Clearly, this presents anger as an emotion that if it is not expressed will eventually "spew out" in your words and hurt both the object of anger and the person feeling the emotion himself.
Blake’s poem uses powerful imagery and visual metaphors in order to convey his message. The “Poison Tree” is first explored in line five, as an extended visual metaphor, comparing hate to a poison tree. This continues for the rest of the poem. In fact, three quarters of the poem utilizes the extended metaphor of a “poison tree.” Blake subverts the natural image of a tree and growth, using this to emphasize how hatred is also a living and growing force. The description of hate as one which “grew both day and night,” and, “bore an apple bright,” illustrates Blake’s idea that like the slow growth of a tree, anger and wrath, when not controlled, can transform into something as mighty and deadly as a poison tree.