After the murder of Duncan, Macbeth struggles terribly with his guilt. He worries that he could not pronounce the holy word, "Amen," when one of Duncan's chamberlains said, "God bless us" (2.2.39, 2.2.40). Macbeth fears that this means that he is damned. Further, he hears a voice cry out that he will never be able to sleep peacefully again because he murdered Duncan while he was asleep and powerless. In fact, Macbeth is so guilt-ridden that he mistakenly brings the murder weapons with him from the room, and when Lady Macbeth orders him to return them, he cannot. He says, "I'll go no more. / I am afraid to think what I have done" (2.2.65-66). Macbeth feels that there is so much blood on his hands that, if he plunged them into the ocean, the blood would turn the whole sea red. Obviously, this cannot be true, but the exaggeration works in the service of another truth: Macbeth's guilt is overwhelming him.
However, Macbeth's guilt fades away quickly. Though he'd felt a great deal of ambivalence regarding the murder of Duncan, he seems to experience no hesitation whatsoever when ordering his next murders: his former best friend, Banquo, and Banquo's son, Fleance. Then, after Banquo's murder, instead of guilt, Macbeth feels only anger that Fleance is still alive. No more worrying about the state of his soul; now he worries only about the security of his throne.
He grows more vicious, certainly, and more ruthless. And in his desperation to maintain his power, Macbeth does become paranoid. After the dinner party at which he sees Banquo's ghost, he tells Lady Macbeth of the lords, "There's not a one of them but in his house / I keep a servant fee'd" (3.4.163-164). In other words, despite their apparent loyalty to him, Macbeth pays a spy in each of the noble's homes to report back to him.
In Macbeth's most brutal act yet, he orders the deaths of Macduff's innocent wife, children, and even servants to punish Macduff for his disloyalty. Macbeth's growing brutality is actually conveyed by the way the murder scenes are portrayed. Duncan's murder takes place off stage; we only see Macbeth's reaction to it. Macbeth becomes more ruthless, and Banquo's murder takes place on stage, but at least his child gets away. Finally, at his most tyrannical and evil, the audience witnesses the murder of a woman and her children on the stage, preventing us from maintaining any form of sympathy with him; at this point, Macbeth is a monster.