There were many things that lead to Macbeth's agreement of murdering Duncan.  The first thing was the three witches, then there was the fact that he wanted to become Thane of Cawdor, and final, to prove Lady Macbeth wrong.

    The first idea of killing Macbeth came about when he encountered the three witches.  Their prophecy had stated that he was to become Thane of Cawdor.  He didn't understand how because Duncan was alive and in good health.  The only way he was to become Thane, was if Duncan was murdered.

    After hearing the prophecies, he became infatuated with the idea of becoming Thane of Cawdor.  But along with his idea of becoming the Thane of Cawdor, others believed that he would be a good Thane.  For example: due to Macbeth's brave actions in the battlefield and the way he handled his rivals, Malcolm felt that he would he a great thane.

    The thing that pushed him even more was, Lady Macbeth was pressuring him.  She felt strongly that he should become Thane of Cawdor.  But Macbeth didn't want to kill Duncan.  So to pursuade him to do it, she would try to intimidate him.  She would treat him as of he was a coward, and would state that if it were her, she would do it just for the love that she had for Macbeth.

    All of these pressures brought Macbeth to the point, where he felt he had to do it.  He felt he had to prove Lady Macbeth wrong and make himself happy.