This is a very subjective question which requires a subjective answer. What this means is that every reader interprets a text, especially poetry, in different ways.
As for the poem "A Poison Tree", one could textually justify that anger is easier to nourish than overcome.
This theology is compounded by the image of the tree. It could remind one of the Biblical saying from Luke 6:43:
No good tree bears bad fruit, nor does a bad tree bear good fruit.
Therefore, a tree rooted in wrath will bear nothing but wrath. Regardless of how adamant a person is, wrath is something which can become so deep-seeded that a simple 'watering' of fears will not insure that wrath will simply go away.
Therefore, the fact that the speaker is glad to see his foe dead at the base of the tree speaks to the fact that wrath is more deeply rooted in man than one could imagine.