When the witches prophesied that Macbeth would be king, Banquo asked for his prophecy. He desired for the witches to give him a prophetic word. He received the following prophecy. "Thou shall get kings, though thou be none." This means that Banquo's children shall be kings. The witches prophesied over Banquo's children.
Sadly enough, the witches' prophecies lead to the death of Banquo. Macbeth knows that only Banquo heard the prophecy or prophecies. Only Banquo knows that the witches prophesied that Macbeth would be king. Macbeth is planning the murder of King Duncan. Macbeth fears that Banquo will suspect him as the murderer. For this reason, Macbeth plan to have Banquo murdered.
Banquo has to die because he knows too much information. He witnessed the witches prophesying over Macbeth. Banquo would and did suspect that Macbeth murdered King Duncan. For this reason, Banquo has to die. He may tell someone all he knows. To himself he says that he suspects Macbeth has murdered King Duncan to get to the throne:
You have it now, King, Cawdor, Glamis, all,
As the weird women promised; and, I’m afraid,
You played most foully for it.
What Banquo means about playing most foully for it is another way of saying that he feared that Macbeth had played dirty to get what he wants. Banquo feared that Macbeth had murdered King Dunacn to get his position. Macbeth was afraid the Banquo would tell all that knew about the witches and the prophecies. Macbeth had Banquo murdered out of fear.