There is something rather cruel in this poem, Blake wrote it against the Anglican church (he was a dissenter), but it also brings a half smile to the mind. There is apparently a drama on television coming out soon called 'The Poison Tree' it was seeing this that bought the poem to mind perhaps the poem explains the drama!
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.
And I water'd it in fears,
Night and morning with my tears:
And I sunned it with smiles,
And with soft deceitful wiles.
And it grew both day and night.
Till it bore an apple bright.
And my foe beheld it shine,
And he knew that it was mine.
And into my garden stole,
When the night had veil'd the pole;
In the morning glad I see;
My foe outstretched beneath the tree.