This is a poem that deals explicitly with the way in which nursing grudges and hatred can result in bearing a kind of fruit that gives full expression to our rage, anger and hatred. It is key to realise the way in which Blakes uses the extended metaphor of gardening to depict the growth of our anger and hatred. Note, for example, the second stanza and the way that the speaker deliberately nurses his grude:
And I watered it in fears,
Night and morning with my tears;
And I sunned it with smiles,
And with soft deceitful wiles.
As a result of all of this cultivation, we are told that an apple appears in the third stanza. Symbolically, this apple is the fruit of hatred and discord, and thus it only seems logical that it is full of pure poison, and when the speaker's enemy steals into the garden to eat it, he dies as a result.
This poem therefore talks about the way in which we nurse anger and just how dangerous and destructive that can be, both to ourselves and to the person we are nursing that anger against. The development and growth of the apple shows the development and growth of anger and how it can reach a size that results in harm to those around us.