Good question. The text is unclear. There is an exchange where Macbeth suggests that he has plans which will augment Banquo's honour and Banquo answers cautiously, "So I lose none in seeking to augment it, but still keep my bosom franchised and allegiance clear, I shall be counsell'd." Did he suspect something? It's a director's call.
Macbeth's soldiers do not kill Banquo, Macbeth hires three murderers to do the job.
Macbeth hires the murderers to kill Banquo for two reasons. Banquo is the only person, besides Macbeth's wife, who knows about the witches' prophecies, which means that he may suspect that Macbeth killed Duncan in order to fulfill their prophecy and become King. Secondly, Banquo is prophesied to become the father of kings, which means that it will be his ancestors, not Macbeth's, who will inherit the throne. By killing Banquo, Macbeth hopes to prevent this prophecy from coming true.
Macbeth does not reveal his plan to kill Banquo and Fleance (Banquo's son) to Lady Macbeth.
Macbeth does not kill Banquo himself possibly because Banquo was his best friend and he could not bear to do it. Furthermore, in not committing the murder itself, he avoids being suspected of the crime.
The three assassins that were hired by Macbeth kill Banquo as he is walking with his son to the palace for a dinner that Macbeth invited him to.
Macbeth's plan was to murder fleance and banquo but it was not accomplished fully because fleance was able 2 escape
Macbeth instructed the murderers to kill Banquo and his son Fleance.
In Act Three of The Tragedy of Macbeth, Macbeth hires murderers to kill Banquo and his son, Fleance. The murderers successfully kill Banquo, but Fleance manages to escape, leaving Macbeth's desire to secure the throne unfulfilled.
Macbeth plans to hire someone to kill banquo
Lady Macbeth wasn't in on her husband's plans to kill Banquo. When she asked what his plans 'of dreadful note' [Act 3 Scene 2 Line 44] were, he didn't want her to know the specifics. But most likely yes, she would have wanted Macbeth to kill Banquo. Earlier in their conversation, she commented that Banquo and his son Fleance were mortal [and therefore killable?].
Macbeth sets an ambush for Banquo with three murderers. The murderers kill Banquo but Banquo's son Fleance escapes.
Banquo is his best friend. Aside from that, Banquo is well loved by many, and if he were to be killed by Macbeth, it would look bad on him. By having the murderers do it, he can mourn Banquo's death and be free of suspicion.