Blake's poem is about being angry with an enemy and getting even with him. The speaker does a lot of things to make his "foe" really seem like an enemy. For example, he tells how he (the "foe") "stole" into his garden, which implies that the enemy has a proclivity (i.e., a tendency toward) for thievery. Ultimately, though, the enemy is the victim of the speaker's anger-apple. While he may admit that he was glad to see his foe laid low, we're left feeling far more conflicted, and more sensitive to the damage our own anger might cause others.
Lines 3-4: The speaker refuses to tell his "foe" about his anger. Something about this person stops him from communicating with him in the same way as he is able to communicate with his friend.
Line 11: The "foe" sees the apple shining. The apple is a symbol for the harmful outcome of hoarding and cultivating negative emotions like anger.
Line 13: The enemy sneaks into the speaker's garden at night. The implication is that he is a thief of some kind. As readers, we're rooting against him.
Line 16: The enemy lies dead beneath the tree. Harsh! The tree is here a symbol of, or metaphor for, the dangerous consequences of festering anger. Although the speaker is high-fiving the world, we're left to take a sober lesson from his ultimately disturbing example.