William Blake, "A Poison Tree"

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 watered 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 veiled the pole;
In the morning, glad, I see
My foe outstretched beneath the tree.

Source: http://writersalmanac.publicradio.org/index.php?date=2005/01/31