The Merriam-Webster Dictionary defines tone as the “style or manner of expression in speaking or writing," the writer’s attitude toward the subject, readers and self. “Macbeth” is a tragedy, so its tone is predominantly dark, elegiac and depressing. “Blood will have blood”, as Shakespeare wrote in Act III, Scene IV of the play, foreshadows the series of murders taking place, which, as a result, stir bleak and sinister feelings among the readers.
Some of the characters, like the Weird Sisters, Hecate and other apparitions, create the supernatural element of the story and produce an eerie tone that amplifies the general sinister tone. Even nature conspires to create a chilling, unnatural atmosphere during the night Macbeth killed the King -- at the time, it was deemed unnatural to kill the king. The witches and the chilling form nature takes are basically external manifestations of a dark, overambitious and cruel soul.
Shakespeare’s tone softens occasionally to make his characters more likeable. The guilt that Macbeth, a merciless despot, and Lady Macbeth, his co-conspirator, feel makes them seem more human, leading the audience to sympathize with them and be drawn to understand their drama. Macbeth’s brutal nature is softened by his hallucinations resulting from his guilt, and Lady Macbeth dies in the end, tormented by their evil actions, whereas the three witches are aloof throughout the play.
In all his plays, including his tragedies or historical plays, Shakespeare includes a comical character that brings a touch of humor to release the tension created by the somber, tragic tone of the play. In Macbeth, this character is the drunken Porter mentioned in Act II, Scene III. It is ironic that the gatekeeper of the kingdom is in a state of drunkenness, which may explain the easy penetration of evil in that realm.