Four lines are enough for Blake to lay out his whole argument. "I was angry with my friend: / I told my wrath, my wrath did end." Then the same situation with a different target: "I was angry with my foe: / I told it not, my wrath did grow." The two pairs mirror each other almost word for word. The only difference is telling versus not telling, and that is what separates a quarrel that dissolves from one that becomes lethal. The rest of the poem follows the second kind of anger once it has been shut inside.
The vocabulary shifts as it goes. In the first line the speaker is merely "angry," but by the second the feeling has become "wrath," a heavier, almost biblical word. By the fourth line it has shrunk to a bare pronoun: "I told it not." That "it" can be hidden like a secret or planted like a seed, and the central metaphor begins there. In the second stanza the speaker "watered it" and "sunned it," and by the third it "bore an apple bright." Nothing announces the change from feeling to plant. The grammar simply slides from one to the other, much as resentment slides from a passing mood into a project.
What makes the image unsettling is how tender the gardening is. Water and sunlight are what keep things alive, yet the speaker waters with "fears" and "tears" and sunshine with "smiles" and "soft deceitful wiles." The rhymes press the point: "smiles" is chained to "wiles," so the friendly face is exposed as a tool. The tears may well be real, since the speaker is suffering as well as scheming. The smiles are not, and the soft, hissing s-sounds of "sunned," "smiles" and "soft" seem to give the game away. Hatred here is fed by politeness as much as by secrecy.
The form works against the content. The lines are short, about seven syllables and four beats, and they rhyme in tidy couplets: friend/end, foe/grow, night/bright, shine/mine. The result sounds like a nursery rhyme or a proverb, something a child could chant. That sing-song regularity gives the story a feeling of inevitability, with each rhyme clicking shut like a lock. It also makes the speaker's calm more chilling, because he describes a killing in the same bouncy music he used to describe a spat with a friend.
The apple carries the most meaning. Nowhere in the text does the word "poison" appear; it lives only in the title. Inside the poem we get an apple that is "bright" and that "shine[s]," and a fruit that looks irresistible. The word inevitably recalls the forbidden fruit of Eden, though here the human being is the one who grows it. The foe "beheld it shine," a slightly formal, almost awestruck verb, and "knew that it was mine." He wants what he knows belongs to his enemy. Then he "stole" into the garden "when the night had veiled the pole." The verb makes him a thief, and the darkness covering the sky suggests a world with its moral light switched off.
The final stanza turns on a change of tense. Until now the story has been told in the past; suddenly it is "In the morning, glad, I see / My foe outstretched beneath the tree." "Outstretched" is a quiet, terrible word, since the poem never says "dead," and it doesn't need to. The speaker offers no regret and Blake adds no moral comment. We are left alone with that gladness. Judging by the poem, the speaker does not seem to notice what he has paid. He spent his nights nursing fear and practising deceit, and he has become someone who can feel glad at this sight.
That is why the poem lasts. In sixteen lines it works as a parable about anger and also as a small study in self-deception. Its message is simple: spoken anger can be dealt with, and swallowed anger grows. Its force comes from the fact that the speaker tells us all of this himself, cheerfully, and trusts us to be the ones who are horrified.