In his famous soliloquy in Act 3, Scene 1, Macbeth expresses his fear regarding his position as the king of Scotland. His main fear is Banquo. He does not trust him, and his mistrust of Banquo is intensified by the witches' prediction that Banquo's descendants will inherit the throne. Macbeth is thrown into panic and anxiety:
For Banquo's issue have I filed my mind;
For them the gracious Duncan have I murder'd;
Put rancours in the vessel of my peace
Only for them; and mine eternal jewel
Given to the common enemy of man,
To make them kings, the seed of Banquo kings!
Macbeth is terrified by the thought that he had to sacrifice so much only for Banquo's descendants to inherit the throne one day. He refuses to accept that the agony he had to go through in order to murder Duncan may amount to nothing.
All these references to Banquo's descendants make us assume that Macbeth will carry on murdering anyone who stands in his way, especially Banquo. Macbeth attempts to rationalize his intention to murder Banquo by wanting to believe that he is a victim in this particular situation. He maintains that killing Duncan entailed forfeiting his soul and losing his peace of mind. Therefore, Macbeth does not want such a sacrifice to come to nothing in the end.