"A Poison Tree" is all about lies and deception. The speaker suns his anger with "soft deceitful wiles," and this anger eventually produces an apple that is deceptively bright and shiny (deceptively because the apple turns out to be poisonous, not made of wax). The speaker isn't the only guilty party, however. In the last stanza, the "foe" "steals" into the garden, presumably in order to steal the apple whose bright, shiny peel must be irresistible. Anger isn't just anger all by itself, then. The poem suggests that its good buddies lies and deceit also accompany it.
"A Poison Tree" implies that negative emotions somehow generate each other. In other words, anger leads to lies, deceit, revenge, and, even in some instances, death. It's a vicious cycle (and not a Harley-Davidson).
Although the enemy essentially breaks into the speaker's garden, we still think that he is the victim. The speaker's deceit and anger seem much worse than just stealing an apple. One point for the dead guy.