He is described by the witches as "lesser than Macbeth, and greater"
This means that when Macbeth chooses to believe the witches and act, he knows
... [DOC] [TLE] Witches Effect in Macbeth :: William Shakespeare - Free Essays [PAR] But with the outside influence from the witches he thinks that that is his destiny 
and he must do everything to ... 'Lesser than Macbeth, and greater.