Macbeth expresses this sentiment in an aside in Act l, scene lll, shortly after the witches' prediction that he would become thane of Cawdor has been confirmed by Ross who has told him that Duncan awarded him (Macbeth) the title after the traitorous Cawdor's arrest and incarceration.
Macbeth is overwhelmed by the accuracy of the witches' prophecy and believes that if this part of the prophecy has come true, it would logically follow that the others would as well. The witches had promised that he would be crowned 'king hereafter.' The idea of becoming king is enormous and Macbeth starts imagining how he would ascend to the throne. He obviously cannot obtain the throne whilst Duncan is still alive. he will, therefore, have to usurp the throne. The only way to do this would be to assassinate Duncan and his sons, Malcolm and Donalbain.
The idea of committing such a horrible deed is too terrible to contemplate and it is for this reason that Macbeth states:
Are less than horrible imaginings:
He means that his current apprehension about the witches and their malevolence does not come close to the dreadful thoughts running through his mind. He is horrified by the idea of even thinking about committing such a monstrous deed. He furthermore states that he is shaken by the idea of considering an act of such heinous barbarity. These thoughts are so powerful that he feels utterly overcome and that his actions are controlled by the idea of doing that which he is too afraid to consider. His frame of mind is such that that which he imagines seems more real than reality itself.
Macbeth is so overcome by his conception that Banquo notices and comments on his enraptured state:
Look, how our partner's rapt.
Macbeth is caught in a reverie and takes some time to regain his senses. This scene depicts a dramatic turning point in Macbeth's thinking and will influence his actions later - actions which will lead him and others to their doom and almost utterly destroys Scotland.