Banquo has children who might inherit the crown whereas "Upon my head they placed a fruitless crown" by which he means the witches have predicted that Macbeth will be the king but will not have children to inherit. ("Fruitless crown" always makes me think of Carmen Miranda for some reason.)
Macbeth kills Banquo in the play and then Banquo's ghost haunts Macbeth.
No logical reason, but then fear is not a logical thing. Although Macbeth says, "My fears in Banquo stick deep," he is not totally clear about why he is afraid of him. Most likely, he is afraid that Banquo knows too much about him, knows what the witches said to him and how troubled he was by it, and as a result may suspect Macbeth of murdering Duncan. He is also annoyed by the prophecy of the "fruitless crown" and the prophecy that Banquo will be the father to kings. To prevent this, Macbeth would have to kill not only Fleance but also Banquo, to prevent Banquo from fathering more children. This prophecy does not threaten Macbeth, yet he feels threatened by it. All of this makes the ball of anxiety which he feels because of the murder focus irrationally on Banquo.
at the banquet, Macbeth sees(maybe hallucinates) a ghost of Banquo, sitting in his place. Macbeth gets freaked out when he sees Banquo because all the people he ordered dead (including Banquo) makes Macbeth feel guilty.
Macbeth's best friend was Banquo. They fought together in battle before Macbeth's rise to power.
Macbeth kills Banquo in the play and then Banquo's ghost haunts Macbeth.
Macbeth sees the ghost of the recently murdered Banquo.
No, Lady Macbeth is not aware that Macbeth has planned the death of Banquo and Fleance.
Macbeth's soldiers do not kill Banquo, Macbeth hires three murderers to do the job.
Macbeth kills him because Banquo starts to work out that Macbeth killed dDuncan
In Act 3 of Macbeth, Macbeth becomes suspicious that Banquo poses a threat to his throne. He worries that Banquo's descendants will inherit the crown, as the witches prophesied. This suspicion leads Macbeth to plot Banquo's murder.
*Banquo. Macbeth's best friend is Banquo, but he is also Macbeth's enemy. After the meeting with the witches, Macbeth finds out that he will become the Thane of Cawdor and the King of Scotland. Eventually, he achieves both of these prophecies. Banquo, Macbeth, and Lady Macbeth are the only people that know about the witches. Banquo becomes suspicious of Macbeth because Macbeth fulfills the prophecy after King Duncan is murdered, and Macbeth acknowledges this fact. Basically, Macbeth doesn't trust Banquo.
It is not known who kills Banquo but it is Macbeth who orders for him to be murdered.