How did talking to his friend about his anger help the speaker in "A Poison Tree" by William Blake?

William Blake's poem "A Poison Tree" is a figurative expression of the relation of the spiritual world with the natural world.
When the speaker of this poem becomes angry with his friend, he communicates his feelings to this friend. As a consequence, his anger is released and the friendship between the two men continues.

I was angry with my friend:I told my wrath, my wrath did end.

In contrast to this amelioration between the friends, the speaker's failure to release his feelings with his enemy causes his "wrath" to increase and intensify, so much so that the poison tree grows as he waters it with his fears and he suns it with "deceitful wiles."
William Blake expresses in "A Poison Tree" the power of the imagination and the spirit. To Blake, the imagination possesses the capability of perceiving the realities of the spiritual world in its expression. On the other hand, the tree becomes symbolic of the corruption that occurs in the soul when a person suppresses feelings. For when the speaker does not release his emotions toward his foe as he does with his friend, and, instead, nourishes his antipathy, the tree grows and eventually produces an apple. This is the apple of cunning and guile--not unlike the apple in the Garden of Evil-- that has sprung from the energy of hate which feeds the tree.
 

By talking to his friend, the speaker is able to come to terms with his anger and let go of his rage. Perhaps the speaker is able to arrive at a mutual understanding with his friend, and this allows his temper to subside.
On the other hand, the speaker is unable to come to a similar understanding with his enemy. He neglects to talk to his foe about what's bothering him. As a result, his anger grows; privately, he waters his anger with his "fears" and "tears." He allows his emotional turmoil full expression in private, but in the presence of his enemy he is all smiles.
Nevertheless, the speaker's anger grows so much that it bears an "apple bright." Here, the shining apple becomes a metaphor for a fully-matured malignancy that is cloaked in an attractive exterior. Unknown to the speaker's enemy, the "apple" is extremely poisonous; it kills his foe after he unwittingly eats it one night. In the end, the speaker can't hide his glee at finding his enemy "outstretch'd beneath the tree" in the morning.