Although Macbeth actually murders Macbeth, it is indirectly Lady Macbeth who murdered King Duncan by convincing her husband that he needs to murder the king. Macbeth does the physical murdering of King Duncan, but Lady Macbeth, it can be said, succeeded in convincing him that he must murder the King. Also, in a sense, the witches are to be held responsible for the murder because it is after the prophecy about Macbeth's succession that these tragedies occur. This ignites Macbeth's ambition to want to be king. The witches expect this when they tell Macbeth about their "prophecy" and they know of the bloodshed and death that it will cause. I believe that every one of these characters play a part in King Duncan's death.
For example, Lady Macbeth
For example, Lady Macbeth