In Macbeth, Banquo suspects that Macbeth has "played foully" for his new position as king. After Banquo learns that King Duncan has been murdered, he suspects that Macbeth has been a part of the murder. Banquo was with Macbeth when the witches prophesied that he would be king. Now, King Duncan is dead and the witches' prophesy has come true. Macbeth is now King of Scotland. Banquo is quite suspicious. He thinks Macbeth has committed murder to gain his new position:
You have it now, King, Cawdor, Glamis, all,
As the weird women promised; and, I’m afraid,
You played most foully for it.
Banquo's next thought is about the witches' prophesy to himself. The witches promised Banquo that his sons would be kings. Now, Banquo is excited about the possibility of the witches being true to their prophesies. Banquo is planning ahead, taking note that the kingdom will be passed on to his own children, not Macbeth's children:
Still it was said [that the kingdom]
would not be passed to your children,
Only that myself should be the root and father
Of many kings. If [the witches] told the truth,
As their speeches shine on you, Macbeth,
Why, by the truths made good on you,
Might they not be my prophecies as well,
And set up my hopes? But I’ll be quiet; no more.
No doubt, Banquo is quite aware of Macbeth having a part in the murder of King Duncan. He has promised that he will not be quiet about it. That is why Macbeth fears Banquo. That is why Macbeth has Banquo murdered.