I am reading all of Shakespeare's plays in the course of a year; this is the second.
I still think of the character Macbeth as a proto-existentialist (in the emphasis on action, the rejection of morality, the weariness with life, the mockery of existence's absurdity), but that doesn't mean I think of Macbeth as a proto-existentialist play (in the same way that King Lear may be). Nature itself balks at Macbeth's crime, and aside from the bloodshed, the drama of the play obviously comes from the psychological conflict and development of the Macbeths.
I imagine the character Macbeth as a terribly difficult stage role to play: the success of a staging of Macbeth must rely heavily on the lead actor's ability and understanding. I thought season two of Slings & Arrows gave this a solid treatment.