The destiny is set for a tragic hero based upon the hero's tragic flaw. The tragic flaw is personality trait that puts the tragic hero on the his (typically a he) tragic path and keeps him on it until death is his price.
In Macbeth, his character flaw is his ambition to the crown (or his lack of spine in dealing with inherently evil women i.e. his wife and witches). So, this tragic flaw of ambition becomes his destiny because once he decides to go for the crown, he commits near patricide in murdering his loyal king, and he most continue on that path until he successfully takes the crown or he fails (which he does).
A quote to best support this snowballing effect of his tragic flaw and decision to let it guide him is shown when Macbeth states, "I am in blood/Stepped in so far that, should I wade no more,/Returning were as tedious as go o'er./Strange things I have in head, that will to hand,/Which must be acted ere they may be scanned.
In those lines, Macbeth admits that his ambition to be king has taken him too far into evil deeds (murders) that he cannot back out without doing more evil, so he may as well push forward.