Power-hungry Paranoia and Other Musings on Macbeth

As I read Acts III and IV, I am consistently amazed at how readily Macbeth believes the witches' prophecy. He says that the only man he fears is Banquo, which is an astonishing turnaround from the beginning of the play, when they were close friends. This fear at the prophecy coming true motivates Macbeth to order Banquo's death. This part confused me; I admittedly do not know much about prophecies, but if you are destined to do something, wouldn't that mean that there is no way to prevent it from happening? I have never been to a psychic, but I assumed that if I went and the psychic told me something I didn't want to hear, that I would have no way of stopping it from happening, because it was already predestined and the psychic was seeing into the future. If the notion of prophecies is similar to the destinies psychics and horoscopes predict, wouldn't Macbeth realize that he cannot change the future? Does Macbeth order Banquo's death out of paranoia, even if he has already been told that eventually Banquo's family will take the throne from him? This reminds of how when ordinary people gain wealth and celebrity they often become paranoid, unhappy, and distrustful of those who helped them get to their high positions, because they are afraid of losing what they have become. Macbeth's paranoias are neverending; he will always fear that someone is trying to usurp him, which is why he orders Banquo's son's death. He appears to be crazy to his guests, which causes Lady Macbeth to be embarrassed and apologize for him.

Lady Macbeth is an interesting character to me. Though she appears morally bereft, it is refreshing to see Shakespeare choose a woman to play such a wicked role, instead of the charming companion role that women typically play in his plays. She seems to be a more confident leader than her husband, although she is definitely more bloodthirsty.