how is macbeth presented as insecure

Macbeth, like Shakespeare's other protagonists, is both an individual and a larger-than-life representative of one or more abstract qualities of human nature. Critics and commentators have often identified "ambition" as Macbeth's chief characteristic. I would take issue with that assessment.
Macbeth is driven by the witches' prophecy, by Lady Macbeth's urging, and finally by the fact that he's gotten in so deep—as he says himself, "so far stepped in blood"—that he has to continue with more killings, as if an uncontrollable chain reaction has been started. His is less a case of ambition than one of "insecurity"—for lack of a more precise term. When the witches are first encountered, Banquo observes Macbeth's frightened reaction to their apparently favorable predictions. Macbeth has perhaps already contemplated murder as a means of advancement but has no confidence in his ability to carry it out, or perhaps he has enough of a conscience to be shaken by the possibility he might do such a thing. But he's not secure enough in his present status as a nobleman and a military leader to resist an evil course that might be suggested to him by Lady Macbeth.
Macbeth's inability to stand up to her prodding is partly due to that insecurity. At the same time, his actions seem almost inexplicable in the context of what we recognize as "normal" human behavior. From our present-day perspective, Macbeth and Lady Macbeth can be seen as sociopaths. The miracle of Shakespeare's dramatic and poetic skill, however, carries us along and engages our sympathy for Macbeth in his tragic fate. When he kills Duncan, Macbeth realizes that he has also destroyed his own life: "Renown and grace is dead.....the wine of life is drawn / And the mere lees is left, this vault to brag of."
When the ghost of Banquo appears, Macbeth becomes hysterical. He is not a coward, but "insecurity" is an apt description of the reason for his behavior, though it's a huge understatement. Macbeth is in the midst of a trance-like dream. It's as if the ground has vanished beneath his feet, and just as in a nightmare, he has no control over himself or the action of the dream. At the end, he straps on his armor to fight Macduff, but less because he wants to defend himself and more because fighting would be no better and no worse than doing anything else. He is trapped in this tale of life "told by an idiot, signifying nothing," as he so eloquently says.
Macbeth is, in fact, a profoundly weak and insecure man, propelling himself into a course of action in which he does not believe and which he knows is wrong. But this is an existential drama in which a man seemingly has no choice but to destroy those around him and himself. We, the audience, identify with him and the story because the play is a stunning parable, presenting a larger-than-life version of the smaller weaknesses and sins to which we all, as insecure human beings, are subject.