EXTERNAL FORCES: Throughout the play, Lady Macbeth tries to convince Macbeth to kill King Duncan, and continues to manipulate him into going through with the murder when he becomes doubtful. In the end, Macbeth is convinced and plots with his wife.
INTERNAL FORCES: When the three witches tell Macbeth that he will be Thane of Cawdor and King, he begins to question and obsess over it. I believe that his constant obsession over the witches declaring him King led to him planning to kill the king. Lady Macbeth only help ed fuel the fire that was already there; his ambition.
Is this really a good idea? Just ambition alone cannot get me through this...
RESISTANCE: Macbeth tries to talk himself out of killing King Duncan be reminding himself of the love the king had for him and his subjects, how murdering someone would set a bad example for others, and the possible consequences he would face if he did kill the KIng.