How is Macbeth relevant to a modern context?
Except for the obvious "lesson" about out-of-control ambition, the play is not really very "preachy"--that is, Shakespeare was not trying to teach a moral lesson or change anyone's behavior. It could be argued that the play warns against equivocation, or interpreting statement personally instead of skeptically; also, modern audiences and readers can find universal wisdom about family loyalties and unity. But the most relevancy comes in the never-changing battle of balancing personal ambition and public welfare, of considering yourself as an individual vs. a member of society--a "one" or a part of something larger. The incident of Dunsinane--"anon, me thought the woods began to move" is a viable metaphor for singularity vs.membership in a "forest"--society.