Poison Tree tells the tale of one man and two different courses of action which he takes regarding his negative feelings. In the first scenario, the man is upset at a friend. However, as he highly regards his friend, he speaks to his friend about his concerns and all is made better.
In the second scenario, the man once more feels upset. This time, as he is now upset at an enemy, he does not share his concerns. Rather, he stays quiet about his anger and allows it to fester. He continues to let the anger quietly grow, even as he deceives his enemy about the true, angry feelings that he harbors.
The anger that he has "watered" grows into a tree that produces an apple. The enemy sees the apple, sneaks into the garden at night, and eats the apple. The man happily enters the garden the next day to see his foe lying there dead, apparently from eating the "poison" apple.
So who are the victims here? Obviously the dead foe who has eaten the poison apple. Other victims? To a lesser extent, one might say that the man himself is a victim. Although he is happy when his enemy perishes, it took considerably energy to keep his anger growing and thriving, an act which might have likewise hurt him in the process.