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:
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.