Resentment only hurts the person who has it.

A Poison Tree
He writes how he was angry with a friend and he told that friend the anger went away. Then he was angry with a foe and did not tell him and his anger just got more stronger, and it kept getting stronger everyday.
The vision here that if you have a resentment towards someone and you fester it and you let that person live in your head rent free, and every time you see that foe and you get angrier and angrier the only person it hurts is you.
You will be better off to tell the person you are angry with as soon as possible.
The poem went on to suggest that the anger grew into a small plant and the plant grew larger till it grew a shiny apple and the plant was now a tree.
The end of the poem his foe got in the "garden" ate the apple became poisoned and died under the tree.
Here it can be implied that this is the garden of Eden, or implies that you finally ended the anger by your foe dying.
This is a rhyming poem. I like rhymes, it just seems to flow better for me when I read it. Rhymes for me seems cleaner and it sounds better, I look at it if it would be a song, how I would hear it. will it ring out and echo.