Macbeth probably would not have killed Duncan if it had not been for the influence of his wife. Macbeth had many misgivings, but his wife kept manipulating him because she was tremendously ambitious. The Three Witches also had some influence on Macbeth, persuading him that what he had to do was inevitable and that he was invulnerable. If Banquo had a wife, she does not appear in the play. If Banquo believes in the witches' prophecies, he does not feel any need to act, since his heirs, according to them, will become kings sometime in the future.
In William Shakespeare's tragic play Macbeth, Banquo remains loyal to Duncan and Scotland while Macbeth does not.
To explain this, one must examine Banquo's character. Most poignantly stated by the witches in act one, Banquo is, by far, the greater man.
Lesser than Macbeth, and greater.
Readers can, therefore, justify that while Macbeth is greater in title, Banquo is the better man by character.
Outside of that, Banquo does nothing to threaten the kingdom of Scotland. Macbeth, fueled by his ambition (and ignoring the morality involved with the recognition of good over evil), decides against letting "chance" crown him and takes the throne. Macbeth does not remain loyal to Duncan.
Banquo, worried that Macbeth took the crown (as seen in act three), states his fears:
Thou hast it now: King, Cawdor, Glamis, all,
As the weird women promised, and I fear
Thou play'dst most foully for't.
Therefore, Banquo, being the better man, realizes that he needs to follow the innate goodness of his heart and remains loyal to Duncan (even after his death).