In the poem "A Poison Tree" by William Blake, how does the speaker feel about his anger and does he regret it in the end? How do you know?

The speaker dissolves his anger with his friend by speaking about it. But when he has anger for his foe, he does not talk about it and it grows. The idea here is that if one suppresses his anger, it will fester and grow, maybe into something even worse. 

In the second stanza, the speaker illustrates how he has developed and cultivated this anger. He uses the metaphor of raising/growing a plant as a way...

The speaker dissolves his anger with his friend by speaking about it. But when he has anger for his foe, he does not talk about it and it grows. The idea here is that if one suppresses his anger, it will fester and grow, maybe into something even worse. 

In the second stanza, the speaker illustrates how he has developed and cultivated this anger. He uses the metaphor of raising/growing a plant as a way of describing how his anger grows. This anger grows to the point that it becomes something quite real: like an apple. This is a reference to the fruit in the Biblical story of the Garden of Eden. The speaker has grown his anger from an emotion and developed it into a object that is meant to tempt his foe. Here, Blake is also criticizing the Old Testament image of God as being more wrathful than benevolent. 

Up to this point, the poem is a description of how suppressed anger grows into something more vile and even more tangible. In other words, when anger is allowed to grow, it will not simply remain as an inner emotion in the speaker. The danger is that it will become something tangible, something real. In the final stanza, the foe "outstretched" seems to imply that his foe is dead. Thus, his anger has culminated in the ultimate act of real, physical violence and murder. But as for the notion of regret, the conclusion of the poem is ambiguous. One way to read the final lines is that he is "glad" to see his foe outstretched. And this could mean he is glad to see the foe dead or he is glad to see the foe in the process of reaching/stretching for the poisoned object that his (the speaker's) anger has become. However, if "glad" describes the morning, the speaker might be saying that in the glad morning, when he is thinking more clearly and with kindness, the sight of his dead foe gives him some remorse or regret. But this is not made clear. The conclusion can be interpreted in either of these two ways. 

Beyond the description of the speaker's own feelings about his anger, this poem is a critique of hypocritical Christian goodness. Blake was critical of people who claimed to be good Christians but actually held hate in their hearts. The lesson is that keeping such hate can lead to actual wrath. Whether or not this critique of Christian hypocrisy ends with regret or happy vengeance is ambiguous. It is therefore, for the reader to decide.