As we discuss in the "Form and Meter" section, Blake's short poem sounds a lot like a nursery rhyme, or a teaching tool to help kids learn a lesson about anger. The rhymes are pretty easy to remember, and the story works to illustrate the consequences of anger. Imagine trying to explain to somebody the potential consequences of being angry all the time. You could say, "Well, my mother always used to say, 'I was angry with my friend: / I told my wrath, my wrath did end. / I was angry with my foe: / I told it not, my wrath did grow."
The poem loves to repeat sounds, and this type of repetition makes it seem like something you learned as a kid. Take the lines "And I sunned it with smiles, / And with soft deceitful wiles" (7-8). Notice the repetition of the S sound here? That's alliteration at work, and it's something that shows up again with all the B sounds in lines 9 and 10. The rhyme and these repeated beginning consonant sounds combine to create a kind of sonic play in this poem, one that makes it sing-songy and just fun to hear or read out loud, like a typical kids' poem.
Speaking of kids, there's one other sound that's repeated a lot in this poem. It's the sound you make when you say "and." That could be because almost half of the poem's lines start with this word. When lines are repeated in the same way again and again, this is called anaphora. To us, it sounds a bit like any conversation we've ever had with a four-year-old: "And then the dinosaur came and he bit the fire truck and then the truck had a wheel fall off and the man came to fix the wheel and dinosaur bit the man on his toe and the man had to run away and the dinosaur chased him and..." (repeat until you zone out). At the same time, though, this repetition lends a sort of urgency to the poem, and lets us know that the speaker really wants to get this story off his chest.
And lest you think the poem is just for the kiddos, don't forget that, really, this is seriously creepy stuff. Very few kids' stories end with someone laughing over the fallen corpse of their enemy. And so, we see that, while this poem has a lot of kid-friendly sounds in it, it's theme is adult-oriented. It's as though the poem lures us in with an approachable sound palate, but then sneaks in this very sobering reflection when we're not looking—just like a foe, sneaking into our poisonous apple orchard. Or something.