"A Poison Tree" by William Blake is a poem that describes what can happen when anger is allowed to grow and fester within a person. The narrator begins the poem by saying that when he was angry with a friend and told him about it, the anger left.
He goes on to say that when he was angry with an enemy and did not talk about it but rather nursed and nurtured it within himself, the anger grew. It finally culminated in him killing his enemy and being happy that he did. The poem was included in Blake's 1794 "Songs of Experience."