The poem Poison Tree is about the ability for people to express their anger in relationships. For example, if you are in a good relationship, you should be able to tell the person when you feel angry. If you are angry and do not share that you are feeling that way, you would be suppressing your anger and that can "eat away" at you. It can make you resentful, stressed out, or your anger could build and then you could explode. In the poem the subject expressed anger to a friend and it was okay. When the subject decided not to express anger with an enemy, it as bothersome and led to murder.