Ahhh! We've changed....

The introduction to the Arden Shakespeare notes that the Macbeths killing of Duncan, Banquo and everyone else didn't seem unconvincing to audiences till around 1900.

If that is correct then it was either:
that the change in our understanding of the human mind leads to a different understand of how people behave (eg Freud, Jung and modern psycho-analysis)
OR
People were just used to barbaric acts - that we in the modern west have trouble believing. But that the Islamic world (eg Taleban), Africa the civil wars accept as common place.