A Poison Tree
The speaker is presenting two scenarios here. In the first, he (we're assuming it's a he) is in a tiff with his friend, a spat if you will. But wait! There's no need to fret. He told his friend about his anger and… guess what? His anger went away. Presto! Ah, the power of communication.
Scenario #2: We get the same basic set-up here. The speaker's mad again, but this time he's mad at his enemy. Will he follow the same route? You bet your bippy he won't. He keeps mum about his anger for his enemy and, well, that anger just grows. The speaker's anger is only heightened by his fears, and his continued deception about his true feelings.
Then, in an odd, metaphorical twist, the speaker's anger blossoms into an apple. Yum! At least the speaker's enemy thinks so. One night, he sneaks into the speaker's garden (presumably for a delicious apple snack), but it doesn't work out so well for him. The next morning, the speaker is happy to see that his foe lying dead under the tree that bore the (apparently poison) apple. Not good.