Just to take a bit of a different angle concerning Shakespeare's Macbeth, first, no evidence really exists in the play that Macbeth was nurturing evil, corrupt thoughts before he hears the witches' predictions. It seems logical, but it's difficult to definitively declare that. Second, if he was, then Shakespeare doesn't show a fair mind being corrupted subtly and gradually over time, since Macbeth was already corrupted when the play opens. You might be able to argue one or the other, but you can't logically argue both.
That said, again, looking at this issue from a different angle, the witches predict only that Macbeth will be king, not that he should assassinate Duncan to get the crown. Macbeth creates that idea on his own.
Once Duncan names Malcolm as his heir, however, Macbeth has little choice, if he ever wants to be king. Malcolm stays behind the lines during the opening battle with his father while Macbeth does the fighting--Macbeth could hardly expect to outlive Malcolm, and even if he does, there's always Donaldbain.
If Macbeth is obsessively ambitious, he has little choice but to kill the present monarch.