Macbeth is afraid of Banquo and rightly so. Banquo heard the witches' prophecy. Macbeth fears that Banquo will suspect him if he murders King Duncan. Since Banquo heard the prophecy that Macbeth would be king, Banquo may suspect that Macbeth has indeed murdered King Duncan to claim his position as king.
After the murder of King Duncan is announced, Banquo does indeed suspect foul play. In Act three, Scene one, speaking only to himself, he states that Macbeth now has it all:
You have it now, King, Cawdor, Glamis, all,
As the weird women promised; and, I’m afraid,
You played most foully for it.
Indeed, Banquo is a threat. He suspects that Macbeth has murdered King Duncan. Now, Macbeth cannot rest knowing Banquo suspects him. In Act three, Scene two, Macbeth is having terrible nightmares, and Banquo is part of the reason that Macbeth cannot sleep:
We have crushed the snake, but we haven’t killed it;
She'll recover, and be herself, while our poor evil-doing
Remains in danger of her poisonous bite.
Only let the order of the universe be disrupted,
Both the real and spiritual worlds suffer,
Before we will eat our meal in fear, and sleep
In the affliction of these terrible dreams
That wake us up nightly. It’s better to be with the dead,
Whom we have sent to their peace to gain our peace,
Than to lie on a bed, torturing of the mind
In a restless frenzy.
In this, Macbeth is saying that he would rather be dead than live in fear of someone like Banquo who knows about the witches' prophecy. Banquo has good reason to suspect Macbeth. Macbeth's only option is to have Banquo murdered as well. Only then will Macbeth feel secure in the throne.
as banquo too knows da secret told by witches macbeth was afraid of him that in one way or other banquo will be more inqusitive about duncans death