Please respond with your thoughts on the following prompt in one paragraph. Imagine how you might argue against someone who thinks differently than...

Technically, there is no personal agency against fate. Oedipus figured this out the hard way. 

With Macbeth, though, there was no specific prediction of how he would become king (as opposed to Oedipus, who was fated specifically to kill his father and marry his mother). The prediction was merely that he would be "king hereafter." Macbeth himself pointed out that this fate did not tie him to any particular course of action: "If chance will have me king, why, chance may crown me, / Without my stir" (I.3). 

This entire question is predicated upon the assumption that fate is real, so I'll answer assuming that. Fate, by definition, cannot be changed. Macbeth's thinking that he was fated to be king did not make it happen; it would have happened anyway. What his thought did was change how it happened. 

This goes into the question of how much personal agency we have against fate: only as much as the "fate" allows. As I've already mentioned, if the fate is specific, it will specifically happen that way--no matter what the fated person thinks or does. In Macbeth's case, however, he made it a self-fulfilling prophesy. Yes, he would have been king, anyway, but he chose to take action to make his "fate" happen sooner.