We get very few details on the setting in this poem. The most identifiable place here is the speaker's garden, which features that bright, shiny anger-apple that lures the foe in: Chomp. Ugh. Thump.
Now, what transpires in the speaker's garden is pretty straightforward (the enemy sneaks in in the dead of night and is found dead there the next morning), but the circumstances deserve a bit of attention. Our first question, for example, is why is the speaker's apple—which is the symbolic representation of his unreleased anger—so attractive to the target of that anger, the enemy? One way to look at it might be that that speaker's anger produced a similar reaction in the enemy. In that sense, his anger attracted the enemy's attention, luring him into the garden.
To continue down this line of thought, the enemy taking a bit of the apple would be his own unhealthy, unresolved anger that's directed right back at the speaker. Just as the speaker grows the poison apple of anger, then, the angry enemy is likewise poisoned.
Of course, all this takes place in the speaker's garden, and it's his apple that's to blame. In this way, the setting subtly reminds us that, while the enemy may have been literally trespassing in the poem, it is the speaker who bears the ultimate responsibility for his foe's death. For shame, creepy gardener, for shame.