It is deceptively simple, isn't it? The sing-songy approach to this is very similar to nursery rhymes or even riddles. It is a riddle, and it is anything but simple when you study the meaning of the words.
You've got four stanzas of rhyming couplets which give it the song feeling.
The appeal? Well, it's the popular and universal theme of anger and revenge. One guy is mad at another, and he didn't talk to his "frenemy" about it. It grew with the sun and the rain and the nurturing (resentment is like this...) it was given. The "frenemy" admired the fruit or apple of the lovely tree and snuck in to get it for himself. Unfortunately, the apple was the product of poison...hatred (which will eat you up inside and destroy you if you let it)...and it killed him. Of course, the owner of the tree was glad to see that his enemy was finally dead.
A morbid subject for a nursery rhyme, but not any more gruesome than the songs about the plague that children sang while playing (ashes to ashes, we all fall down).
Blake probed the psychology of repression long before Freud came on the scene. Anger in itself is not bad, Blake suggests, but anger held in, watered in “fears,” and repressed grows a poison apple that has the power to do irreparable damage. This poem does not so much warn against wrath as warn against wrath unexpressed, unexplored, unchecked, and remaining hidden. It hides behind a hypocritical surface of smiles and deceit. It festers and turns poisonous, lethal. The childlike quality is reinforced by the simple sentences of the first stanza, by the frequent use of a simple and instead of a stronger logical link, and by the repetition of the I seven times. The gardening symbolism may also seem deceptively simple and childlike—until we sense echoes of the fatal apple in the Garden of Eden and of the first fratricide, the killing of Abel by Cain.