Macbeth has committed great deal of evil by the end of Act III. Fearful of the prophecy that Banquo "shall have kings," Macbeth has ordered the deaths of Banquo and Fleance to destroy Banquo's line. This order came from Macbeth alone; he never consulted or confided with Lady Macbeth beforehand, which shows how isolated he has become. Macbeth had hoped the deaths of the two would help him achieve the security he desires, but Fleance escaped the murder attempt, and Banquo's ghost entered during the Act III, Scene IV dinner party, wreaking havoc on Macbeth's state of mind, in front of all of his guests. Macbeth was so out of control in the scene that even Lady Macbeth's attempts to contain him had little effect.
After Macbeth has made the scene so incredibly awkward that the guests leave, Lady Macbeth attempts to reason with him. It is at this time that he truly reveals his state of mind. His paranoia causes him to say that "Stones have been known to move, and trees to/ speak./ Augurs and understood relations have/ By maggot pies and choughs and rooks brought forth/ The secret'st man of blood" (3.4, 154-157). By this Macbeth means that his secret murders will be revealed in some way, perhaps by nature itself. He then begins to wonder where Macduff is, as the noble did not come to the party. This creates even more paranoia, as it leads Macbeth to begin thinking about what other entities might be conspiring against him. He then tells Lady Macbeth that he is going to go visit the Weird Sisters again, to find out what will happen next. Finally, he gives the audience a true window into his state of mind, saying:
For mine own good,
All causes shal give way. I am in blood
Stepped in so far that, should I wade no more,
Returning were as tedious as go o'er.
Strange things I have in head that will to hand,
Which must be acted ere they may be scanned. (3.4, 167-172)
In other words, everything will move for his own good. He uses the metaphor of a pool or lake of blood to say that he has done so many terrible things that turning around would be just as difficult as continuing to move forward by doing more terrible things. Finally, he suggests that his past habit, of allowing his conscience to sometimes hinder him, will occur no more. He will act the moment he thinks of something, so that there will never be a chance for anyone to suspect him before any terrible acts are committed. When Lady Macbeth attempts to coax him to bed, telling him that he needs to rest, he agrees but states "We are but young in deed," meaning there is much more to come.
By the end of Act III, there is no turning back for Macbeth. He has done many terrible things, but he has resigned himself to doing even more. He will not rest until he feels safe in his position, but because of his paranoia he will never truly feel safe. Nor will he ever truly be.
Act III marks a turning point in Macbeth's personality and state of mind. The once sympathetic and noble Macbeth has become dark, sinister, paranoid, and filled with blood lust. In other words, he has shifted from 'good' to 'evil' in the minds' of the audience. He is paranoid about his ill-gotten powers, and is using ruthless violence to protect himself out of desparation.
Of particular note, Macbeth orders the death of Fleance (Banquo's son). This would certainly illustrate Macbeth's state of mind. Ordering the killing an innocent child indicates a complete breakdown of character.