How does the poem "A Poison Tree" by William Blake support or negate the proposition that anger tends to make people irrational?"A Poison Tree" by...

This excellent poem clearly concerns the theme of anger
and how when it is not dealt with it can become intensely destructive. In relation to
your question, it might be helpful to look at the second stanza, which, through an
extended metaphor, discusses the way that the speaker dealt with his anger and nurtured
it, comparing it to tending a plant:

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And I watered it in
fears,

Night and morning with my
tears;

And I sunned it with
smiles,

And with soft deceitful
wiles.

Note the imagery in
this stanza and the way that the speaker tends his anger as he would a tree. The speaker
is described as feeding his anger with tears, fears and deceit, which, we could argue,
is not the most rational way to approach his anger. Clearly this could be used as
evidence to support your proposition. Due to the anger that the speaker harbours inside
of him towards his foe, he expresses a range of emotions indicating the way that he is
looking his rationalism and becoming increasingly irrational. This of course relates
back to the key theme of the poem in the way that anger that is left to simmer and
fester ends up hurting both the object of that anger and the person nursing those
feelings as well.