I blame Lady Macbeth.
And c'mon, if some witches you believed in said you could not be killed (okay, so there was a loophole) you wouldn't act any differently?
I agree it isn't clear cut, nature versus nurture. As a classic tragedy, echoing Greek drama, I imagine there has to be an innate flaw in Macbeth. He has to have an intentional hand in bringing about his downfall. If we accept that it was forces beyond his control then there is no hubris-then-fall, no tragedy, just "a dingo ate my baby!"