Are you looking for a common message in the two plays? If so, I would start by examining the source of the tragedy in both stories. Both plays end in a number of deaths including those of the main characters (Romeo and Juliet, Macbeth and Lady Macbeth). So which flaw or flaws of the characters caused their deaths and the deaths of so many around them?
In the case of Macbeth, both Macbeth and his wife are done in by their ambition. In order to get (and keep) what they want, they'll stop at nothing including people to whom they should be loyal (Duncan), former friends (Banquo), and even small children.
In the case of Romeo and Juliet, the characters are doomed by both their haste and their selfishness. In order to get what THEY want, they act without forethought of the consequences of their actions.
So if you're looking for a message common to both plays, you might say that the characters are doomed by pursuing what they want at the cost of others' lives and happiness.
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