Banquo and Macbeth together foght the battle which led him to win the king's heart. On their journey back home, they cross a deserted land where the encounter the three wicked sisters, the witches. an encounter the wicked sisters were looking foward to. This was an opportunity for the witches to cause Macbeth bring his own downfall.
this was not planned by some nobleman or anyone, but this was an act of pleasure for the witches. The wiches greeted Macbeth with three names: Thane of Glamis, Thane of Cawdor and The king ( a title which grabs Macbeth's attention, and causes the begininning of his restless mind and soul). At the same time they also prophezise that Banquo would be the father of the line of descendants to the throne of Scotland. Since this encounter, Macbeth always remains restless and secretive, however he avoids being doubtful to suspiscion.
After the murder has been committed, and macbeth's coronation is over, Macbeth becomes the king. he is now bothered by the witches' prophecy about Banquo> and he feels guilty thinking that" had he ruined his life for someone else?/Had he spilled blood on his hands for someone else's advantage?/ had he sold himself to Satan just so that someone could reap benefits of it?">
In order to avoid the prophecy from coming true, Macbeth secretly and cleverly plans Banquo and Fleance's Murder. What is to be noted here is that, how the sweet benevolent, simple and loyal nobleman becomes such a hypocrit and a cunning man,who is now ready to take his own companion's life for his own betterment/just because he thinks that his companion stands a threat in his well fed future.
however the fact is that Macbeth will always experience the huge robes slipping down his dwarf like body, for they rightfully never his.
(BASIC POINTS REQUIRED TO BE EXPLAINED FOR A 20 MARK ANSWER= SECTION B. ISC)