William Shakespeare's play Macbeth has a very dark mood. Plays, during this period, were placed into one of two categories: comedies and tragedies. Macbeth, in all accounts, is a tragedy.
Macbeth qualifies as a tragedy based upon the following characteristics:
1. The tragedy follows one main character: the tragic hero. The tragic hero possess hamartia (or a tragic flaw which leads them to their downfall). Macbeth follows its namesake, Macbeth, who possesses a tragic flaw given his ambition is given precedent over the concept of right and wrong.
2. The tragedy contains tragic events and suffering. The play contains much suffering (as seen in the murder of Duncan, Banquo, and the mental deterioration of Macbeth and Lady Macbeth).
3. Many tragedies contain elements of questionable conditions. In the play, hallucinations, insomnia, and the supernatural play a major role.
Therefore, given that the play contains these major elements of the tragedy, the play could be nothing less than dark based upon the murder, treason, use of witchcraft, and diminished mental capacities.