In Act 1, Scene 7, Macbeth delivers a soliloquy in which he considers all of the reasons he has not to commit the murder of Duncan as well as the one reason he has to move forward with this crime. He says, "I have no spur / To prick the sides of my intent, but only / Vaulting ambition, which o'erleaps itself / And fall on th' other--" (1.7.25-28). So, while he has a great many reasons to justify not committing the murder, he has only one reason to go through with it: ambition. However, when Lady Macbeth enters the room, just at this moment, he immediately says to her, "We will proceed no further in this business" (1.7.34). Thus, it seems to me that ambition alone is not enough of a catalyst to prompt him to take action because almost as soon as he identifies it, he tries to cancel their plans.
It is only when Lady Macbeth wounds his pride, mocking his bravery and manhood, that he relents. She insists that, if he will not murder Duncan to take the throne, he will have to "live a coward in [his] own esteem," and that when he made the promise to her that they would proceed with their plan, "then [he was] a man" (1.7.47, 56). She implies that he is not a man if he breaks this promise and swears that she would be willing to kill her own child if she had promised him to do so. Thus, it is only when his pride is wounded that Macbeth finally commits fully to the plan to murder Duncan, and so I argue that pride is his true tragic flaw, not ambition.