The murder of Banquo shows the change in Macbeth's character and status in several ways. First, Macbeth has made the transition from warrior to king. Before this point in the play, he did his killing himself, both in battle, and in murdering Duncan and his guards. Now he has assassins, whom he engages to kill Banquo, and later Macduff's family, on his behalf.

With this change in status comes a similar alteration in Macbeth's mindset. He debated with himself long and hard over the murder of Duncan, and was appalled at his crime when he had committed it. Now, murder is easy for Macbeth, and it is his preferred solution to any problem, even an imaginary one. He is right to think that Banquo suspects him of treachery, but there is no reason to suppose that he will do anything to harm Macbeth, at least in the immediate future. Nonetheless, Macbeth's first thought is to murder his closest friend. He has become altogether ruthless, even in his attitude to his great friend and comrade in arms.

Kingship is said to be a lonely position, and the guilt of recent regicide must exacerbate this loneliness. It is striking how isolated Macbeth has suddenly become at the point when he decides to murder Banquo. Not only does he decide to kill his best friend, but there is not indication that Lady Macbeth, who spurred him on to kill Duncan, is even aware of this latest murder. From this point in the play, Macbeth acts alone.

To be thus is nothing,
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be feared. 'Tis much he dares,
And to that dauntless temper of his mind
He hath a wisdom that doth guide his valor
To act in safety. There is none but he
Whose being I do fear, and under him
My genius is rebuked, as it is said
Mark Antony’s was by Caesar. He chid the sisters
When first they put the name of king upon me
And bade them speak to him. Then, prophetlike,
They hailed him father to a line of kings.
Upon my head they placed a fruitless crown
And put a barren scepter in my grip,
Thence to be wrenched with an unlineal hand,
No son of mine succeeding. If ’t be so,
For Banquo’s issue have I filed my mind;
For them the gracious Duncan have I murdered;
Put rancors in the vessel of my peace
Only for them; and mine eternal jewel
Given to the common enemy of man,
To make them kings, the seed of Banquo kings!
Rather than so, come fate into the list,
And champion me to th' utterance.

Macbeth's initial reaction to Banquo's murder is to rejoice, praising his assassins for their work. But he is obviously torn by guilt, as his visions of his dead friend's apparition suggest. The most important change following this murder, however, is that he no longer turns to Lady Macbeth for advice or motivation. She has lost her role as the driving force behind his actions. Immediately following the banquet where he sees Banquo's ghost, Macbeth resolves to go see the witches. The visions they conjure for him give him a sense of overconfidence that causes his ambition, and his capacity for murder, to spiral even further out of control. In this sense, his murder of Banquo (and attempted murder of Banquo's son Fleance) is a major turning point in the play.

Well, one way is that Shakespeare uses Banquo's murder to develop Macbeth's characterization in terms of his growing ambition and subsequent slipping morals.  With Duncan's murder, the audience really felt Macbeth's indecision and his struggle over his ambition versus his sense of morality--that killing Duncan was murder and wrong.   With Banquo, Macbeth takes a much more active role; his wife does not have to  prod him to carry out the murder.  In fact, Macbeth plans the murder on his own and hires the murderers to complete the act.  Banquo's murder really showcases the fact that Macbeth feels like he is taking charge of his own destiny by removing all obstacles, like Banquo and Fleance, for example, that stand in his way of the throne.  Moreover, Macbeth's reaction to seeing Banquo's ghost in Act III Scene 4 at the banquet shows the reader that although Macbeth seems more comfortable planning murder, his guilt still plagues him.  He is becoming more paranoid in that regard, consumed with suspicion, fearful that his guests suspect him as a murderer.