A Poison Tree
The speaker wants to share with us. Isn't that nice? He'd like to share with us how he killed his enemy. Dude, we said share, not scare.
Still, scaring is really the point of the speaker's story in the first place. He's relating his own mistakes in order to help us avoid the same kind of misstep in future. And he does so in a very effective, surprising way. He's not into saying things like "Oh, what a fool I was!" Nope. Instead, he takes you through the process that led him to poison his foe with the symbolic anger-apple and, importantly, allow us to the be the one to cast judgment on him.
Think about that for a second. Most people, when they want you to learn from their mistakes, are pretty explicit in telling you how terrible they feel now, and how awful their mistake was. In a weird way, this kind of judgment—while meaning to convince us of the importance of the lesson on one hand—stops you from judging them yourself on the other. The person really is judging themselves for us and saying, "Don't be an idiot like I was."
We get none of that in this poem, though. The speaker leaves us with this chilling note: "glad I see / My foe outstretched beneath the tree" (15-16). Uf. That's extra-creepy, if you ask us. And, of course, that's precisely the reaction that the speaker is going for. By not denouncing his own actions, the speaker is selfless enough to let us do that to him. As a result, his lesson is all the more effective, because it invites us to draw our own conclusions about him, rather than give us his own pre-packaged judgments.
That's really quite big of the speaker. It seems that he's so invested in us taking a lesson from his mistake that he's willing to use himself as the most shocking example of how not to behave. Thanks for the tip, creepy guy!