After hearing the prophecy of the witches in Act One (of Shakespeare's Macbeth), Macbeth faces a very dire internal conflict. (Internal conflict is the one a person fights within themself in regards to their morality and knowledge of right and wrong.) Macbeth, soon after hearing the prophecy, decides that only chance can crown him.
Unfortunately for Macbeth, the conflict between his ambition and his sense of right and wrong come to a head. Macbeth wants to be King, but he is not sure if murdering Duncan to gain the crown is worth the murder. Macbeth's ambition drives him to claim the crown, but his morality forces him to consider what is right and what is wrong. In the end, Macbeth's ambition wins out over what is right.
In the beginning, Macbeth acknowledges that it would not be right to murder Duncan to gain the crown. It is not until he is pressured by his wife, Lady Macbeth, and his ambition grows that he begins to really desire the crown, regardless of the morality of murder.