This is a very interesting question, and the key to answering it, I would suspect, is to first define what a good king actually entails. It's tricky, because different generations have different answers to this questions (times have changed a lot since Shakespeare's day. Especially in the modern era, by which time, in the western world at least, monarchy has largely declined to a ceremonial position).
In any case, I'd suggest that good monarchs act to ensure the stability and security of the State. Whether its a Constitutional Monarch or an Absolutist Monarch, this has remain a central theme of Kingship. Therefore, I'd suggest that any qualities a monarch might possess have to be viewed within that context.
With this in mind, I don't think there is any quality which would qualify Macbeth for being a good King. Quite on the contrary, I'd suggest that Macbeth is first and foremost a usurper. He wins the throne through murder and proceeds to plunge the realm into unrest. He is cunning, and he's...
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