What is the tone of the poem "The Poison Tree" by William Blake?

That's a great question.

There is a layered or ambiguous tone to this poem.

The content of a work is, in some works, distinct from the tone. That isn't really the case here. The poem's content is marked by so many angry words--"wrath," "angry," "foe," etc. You can't help but hear part of the tone as, well, angry.

However, the tone itself is calmer, more distant, rational, and measured. The regularity of the rhyme, and the simplicity ("friend" and "end," "foe" and "grow," etc.) makes it more playful and singsong. The combination creates a tone of reflection, but also perhaps of denial and repression.

Take the narrator at his word: he hates this person, and considers him his enemy. If that's the case, then the ability to speak in so calm and measured a fashion means he is fundamentally divided, even hypocritical about his emotions.