Answer:
The two factors that influence Macbeth's decision were: 1) The witches and 2) Lady Macbeth
Explanation:
There were several factors that influenced Macbeth to kill Duncan, but the two main factors were the witches, who appear in act 1 scene 3, and Lady Macbeth, in act 1 scene 5.
The witches promise Macbeth that he will ultimately become the "King of Scotland". Since the king was still alive, Macbeth wondered when he will be king. This instilled in Macbeth the thought of killing the king, since he believes that the evil witches were foretelling the inevitable outcome.
Lady Macbeth comes in later, and when she hears the prophecy, she strengthens Macbeth's resolution to kill Duncan. Lady Macbeth, through her manipulative powers, could convince Macbeth that he would only become king by executing king Duncan. She also stays that Macbeth would be a coward if he does not kill the king.
So Macbeth succumbs to both influences, and also to his pride, and eventually kills Duncan.
