How does Macbeth's state of mind changes on Act 3 Scene 1 from the last act
Act II is where Macbeth begrudgingly agrees to kill his King, Duncan. Remember that Lady Macbeth has to force him to complete the task by questioning his man hood and pushing him into the room. Once he completes the task, Macbeth fills guilty and cannot bear to return to the room where Duncan's body is to place the daggers in the guards hands.
By Act III scene I, Macbeth seems to have recovered from his fears. As Banquo points out the witches predictions have come true for Macbeth, Macbeth calmly asks his "friend" his plans for the day.
We should have else desired your good advice,
Which still hath been both grave and prosperous
In this day's council; but we'll take tomorrow.
Is't far you ride?
Which makes it sound as though he has an important issue to discuss with Banquo, but is in reality just a means of testing how far Banquo is planning on traveling.
Macbeth has decided, without the encouragement of his wife, that to keep his power, Banquo must die. Macbeth has changed from a weak man who does what his wife tells him to do, do a man able to plan the cruel murder of one of his best friends and his son.