In a sense, this is comparing apples with oranges. Blake's "A Poison Tree" is a short lyrical poem, and Romeo and Juliet is a play. One was written in the late eighteenth century and the other over two centuries earlier. While Shakespeare was probably a fairly traditional Christian, Blake in this as in his other poems was concerned with inventing his own religion. Shakespeare's drama is about two young lovers, and Blake's poem focused on the nature of all human relationships, not love per se.
Although you could make arguments about using Blake's approach to the nature of feuds in general to the feuding families in Shakespeare's play, the comparison seems rather random. It is usually better to start with a thematic point of comparison and then choose examples than to choose two works at random to compare. For Blake, a better comparison would be Milton, as Blake was fascinated by Milton; you could compare "The Poison Tree" with Milton's account of Lucifer's fall. For Romeo and Juliet, you might want to compare it to other stories in which families oppose the wishes of two young lovers or stories of family feuds; As You Like It or The Merchant of Venice might be good choices.
To me, it is about the dangers of letting feuds fester. If you don't address a problem, it gets worse and worse until it destroys everything around it. Romeo and Juliet's families feuded for years, and it took their deaths to put things to bed.
Great question. Well, let me start the ball rolling by pointing towards the way in which "A Poison Tree" is all about the way in which anger and hatred that is not expressed and that is nursed and cultivated results in death and tragedy for both the person bearing the grudge and the person who is on the receiving end of it. We can clearly see the truth of this in the play, as the Montagues and Capulets are busy nursing their grudges against each other and will not let them go, but instead cultivate their hatred like the speaker in the poem.