Literary critics don't call the play Shakespeare's "darkest tragedy" for nothing. It opens with three witches conjuring on a heath amidst thunder, lightening, "fog and filthy air," which establishes a dark and murky atmosphere that permeates Macbeth – a play that's full of sinister prophesies, murder, and general wickedness.
Even the humor and seemingly light hearted moments are bleak. In Act II, Scene iii, the Porter famously jokes about who could be knocking at the doors of Macbeth's castle at such an hour. He goes through an extensive comedy routine as he imagines what it would be like to be the porter at the gates of hell. (Pretty busy, apparently, because there's so much evil in the world. Then again, notes the Porter, Macbeth's castle is much too "cold" to be hell.) The joke, of course, is that Macbeth's castle is a lot like hell, especially since Macbeth has just murdered Duncan while the king was sleeping. Cue the uncomfortable laughter. Later in the play (act four, scene two) Macduff's young son and his wife crack jokes about how Lady Macduff will have to go to the market to "buy" a new husband, since hers has seemingly abandoned her. And just as we're enjoying a few chuckles, a couple of murderers enter and stab Macduff's son in the guts. So there's that.