Macbeth intends to secure himself as King of Scotland by all means possible, once he realises that it may be within his grasp. The witches have made it a very real issue for him and his biggest fears, apart from being discovered, are having anyone stand in his way.
After Macbeth's initial decision to "proceed no further" with Duncan's murder, he is soon persuaded by Lady Macbeth to commit this heinous deed.
Banquo questions Macbeth's intentions in his quest to be king after Duncan's death - "Thou play'dst most foully for't" - but does at least wonder about the prophesies and the claim that he will be "the root and father of many kings."
The realisation that Macbeth may have actually killed Duncan on behalf of Banquo and "set me (Banquo) up in hope" sends Macbeth on a mission to remove any such threat as otherwise: "To be thus is nothing,/ But to be safely thus."Macbeth will not be happy and feel secure until the menace that Banquo represents - and his sons - is removed.
Whilst Banquo does not trust Macbeth, he does not consider that Macbeth could be responsible for Duncan's death, despite his suspicious behavior. Banquo is an honorable man and it is inconceivable to him. When he is stabbed, he is only concerned for his son who manages to escape.
It is reflective of the effect of the witches as both Macbeth and Banquo were made grand promises by the witches but Banquo never for one minute considered taking matters into his own hands. Banquo saw the prophesies as revealing the potential for the future whereas Macbeth saw it as his absolute destiny. The "vaulting ambition" of Macbeth overtook all reason and decency.