Which is the best example of a tragic impulse from shakespeareâs the tragedy of macbeth?

I believe it is the witches' prophecy, at the very beginning of the tragedy. It fuels Macbeth's ambitions, so that the drama may unfold.

A tragic impulse is an event which inspires a character to undertake morally problematic actions with destructive consequences. Eventually, it leads to the character's downfall.

Thanks to the prophecy that Macbeth would be the king but Banquo would father kings, Macbeth realizes that he has no choice but to kill king Duncan. That is the moment when his tragedy begins, and when he enters the magic circle of crime. Later on, he has to have Banquo killed, to prevent him from fathering kings.