In Macbeth, Banquo and Macbeth are already beginning to be suspicious of each other before Macbeth sends the murderers to kill Banquo.
Macbeth and Banquo were brothers in arms and friends of a sort. Both were soldiers that worked together.
First of all, Banquo was there when Macbeth got the prophecy from the witches that he would be king, and the prophecy also said Banquo’s sons would be king. Macbeth complains that he was given a “fruitless crown” because Banquo’s sons and not his will be king (Act 3, Scene 1, enotes etext p. 42).
When Macbeth finds out that Malcolm was named heir, he is annoyed. He says that Malcolm is a step he must “fall down, or else o'erleap” because it lies in his way (Act 1, Scene 5, p. 18). Basically, Banquo becomes the next step.
Banquo is well aware that he stands in Macbeth’s way. He is also beginning to wonder if he is a threat to Macbeth, because he suspects that Macbeth killed Duncan to become king.
Thou hast it now: King, Cawdor, Glamis, all,
As the weird women promised, and I fear
Thou play'dst most foully for't (Act 3, Scene 1, p. 40).
Banquo is concerned that Macbeth might do something to him. His concerns are justified because Macbeth is indeed planning his murder. Macbeth says of Banquo, “tis much he dares” (p. 42), because he believes that Banquo might move against him.
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 fear'd. (Act 3, Scene 1, p. 42)
Basically, at this point the two old friends are both afraid of the other. Each one is suspicious, and each one’s suspicions are justifies.