William Blake was born in 1757 and was originally an engraver. He began adding text to his engravings in the form of poems and he was interested as much in the presentation of poems as the poems themselves. In 1789 he published an illustrated set of poems called Songs of Innocence and in 1793 followed this with Songs of Experience (from which A Poison Tree comes). The following year, he combined these two sets of poems, publishing as Songs of Innocence and Experience Showing the Two Contrary States of the Human Soul.
The first set of poems is, therefore, generally hopeful and positive while the second set tends to be more negative and pessimistic.
Blake was a deeply religious man and this shows in the moral nature of his work. His poetry was not really well-regarded during his own life. Today he is regarded as a man ahead of his time and he is now thought of as a major poetic writer.