You can discover a lot about a poem by comparing it to one by another author that deals with a similar subject. You could compare features such as theme, form, structure, rhythm, language and figures of speech.
The key thing to do when comparing poems is to note the points where they are similar and the points where they differ. You could make a list noting similarities and differences between the two poems.
Comparison of 'A Poison Tree' by William Blake with 'Envy' by Mary Lamb
Both poems appear to be autobiographical.
Sentence breaks occur within lines and the sense carries over into the next line.
They set out to make a moral point to the reader.
Both are about core human failings – in religious terms Wrath and Envy are two of the Seven Deadly Sins.