"A Poison Tree
" is already figurative; I assume you mean to ask how its figurative language might reasonably be interpreted.
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.
Here, Blake speaks of how we deal with our anger. We don't want to remain angry with friends and are more willing to talk to them, so we stop our anger there instead of leaving it to fester, or "grow" as he says, a word used ambiguously here, referring to how our anger intensifies; this word allows Blake to turn "wrath" into a tree, though, as it is "growing."
And I waterd it in fears,
Night & morning with my tears:
And I sunned it with smiles,
And with soft deceitful wiles.
In the image of the Tree of Wrath (as opposed to the Tree of Life or the Tree of Knowledge of Good and Evil), Blake shows how we make our anger worse, turning it to hatred. We add our fears to it and we add deceit (since we aren't willing to admit we're angry). Here, we have the Tree of Wrath being nourished with negative behaviors.
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.
As we add other emotions to our anger, it only grows. In this case, the tree eventually produces a shiny bauble (an apple, reminiscent of Eve's temptation in the Garden of Eden) which entices his enemy, who does not know that the day he eats of it, he shall surely die.
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.
So his enemy sneaks into his garden, knowing by now that he hates him anyway, and steals the apple. The originally angry man is by now full of hatred and is pleased to see that his foe fell for it and is no longer a problem.
This is the end result of our anger if we don't deal with it immediately. Note how un-godlike it is. When God discovered his creations had eaten of the forbidden fruit, he was angry and disappointed, but not pleased. Man, comparatively, has an infinite capacity for hatred.