When the play begins, Macbeth and Banquo are close friends and comrades in arms. They have just fought together to vanquish Duncan's foes. They are both equally surprised by the witches' prophecies, although Macbeth reacts more strongly to the witches' words than Banquo.

By Act III, scene 1, their relationship has changed dramatically. Once Macbeth becomes king, he and Banquo can no longer be bosom buddies, but worse than that, each has growing suspicions of the other. Banquo increasingly fears that Macbeth murdered Duncan. Macbeth increasingly fears Banquo knows too much. As Macbeth becomes harder-hearted and more disappointed in the way kingship has turned out to be not all he had thought it would be, he grows increasingly resentful of his former friend. Macbeth feels he has taken all the risks to get the crown, but the witches' prophecy states that Banquo's descendants will sit on Scotland's throne. That doesn't seem fair to Macbeth. At this point, it is easy for him to order his former buddy assassinated.

In killing his friend, Macbeth fulfills his own prediction that the bloodshed would only get worse after Duncan. Macbeth is increasingly living in a sea of blood. He is also increasingly acting as a tyrant and isolating himself from the people who might have had a good influence on him.

Initially, Macbeth and Banquo are loyal soldiers and close friends. The two characters fight side by side in battle and courageously serve King Duncan. They both receive enigmatic prophesies but react differently after listening to the Three Witches. Macbeth immediately allows his ambitious nature to take over, while Banquo is more cautious about the intentions of the witches. Banquo even warns Macbeth that the prophecies may be nefarious and have disastrous results. However, Macbeth does not listen to Banquo and is convinced by his wife to murder King Duncan. Following King Duncan's murder, Macbeth becomes distant from Banquo and begins to view him as a threat to his throne. Macbeth does not want Banquo's sons to inherit his title and begins to plot against his friend. Banquo also becomes aware that Macbeth may have played a significant role in Duncan's death and becomes suspicious of Macbeth. By act 3, Macbeth has commissioned assassins to kill Banquo and his son. Macbeth essentially goes from being close friends with Banquo to being his enemy by act 3. 

At the beginning of the play, Macbeth and Banquo are both soldiers in Duncan’s army.  Both are noblemen, and they are friends and colleagues.  By the third act, each suspects the other of standing in his way, and Macbeth kills Banquo.

In the first Act, Macbeth and Banquo are on fairly equal terms.   When they meet the witches, they are told that Macbeth will be Thane of Cawdor and then king, and Banquo’s sons will be kings.  Neither man appears to take the prophecy seriously, and Banquo seems more disturbed than Macbeth. 

When they get the news that Macbeth has been promoted to Thane of Cawdor because of his bravery in killing the traitor who previously had that title, Banquo’s response is “can the devil speak true?” and Macbeth asks why he is dressed in “borrow'd robes” (Act 1, Scene 3).  Yet Macbeth seems disturbed by Banquo’s reaction to the witches, and asks him if he doesn’t want his sons to be king.  Banquo replies that Macbeth’s interest in the crown seems to have been spurred, but he is concerned.

But ’tis strange; 
And oftentimes, to win us to our harm, 
The instruments of darkness tell us truths, 
Win us with honest trifles, to betray's(135) 
In deepest consequence— (Act 1, Scene 3).

When Macbeth and Banquo get the news, Macbeth is already wondering if Banquo is supporting him. 

Two truths are told

As happy prologues to the swelling act

Of the imperial theme!” (Act 1, Scene 3).

When he says this, he is NOT talking to Banquo. In fact, Banquo notices that he seems to be lost in thought.  Banquo already is getting concerned about the effects of Macbeth’s ambitions.  At this point, an interesting conversation takes place inside each of their heads, but not between them.  Macbeth thinks to himself “chance may crown me Without my stir” but Banquo is thinking that “new honors come upon him, Like our strange garments” (Act 1, Scene 3).  Macbeth thinks he might become king without doing anything else, and Banquo thinks Macbeth’s promotion does not fit very well.

When Macbeth acts, Banquo has already become suspicious of Macbeth's rise to king.  He is afraid that Macbeth has “play’dst most foully for’t” (Act 3, scene 1).  He notes that no one else seems to realize what Macbeth has done.  He also wonders if his prophecy will come true, and his sons will be kings.  This is not ambition on his part, but fear.  He knows he is a threat to Macbeth.

Macbeth is also suspicious of Banquo.  He is afraid that if his prophecy came true, so might Banquo‘s.  He is annoyed with his “fruitless crown” and realizes that he needs to get rid of Banquo if he is going to continue his line as king and have sons who are kings.  Macbeth decides the only thing to do is kill Banquo.