This question has already been answered. Here is a link for you: http://www.enotes.com/macbeth/q-and-a/how-does-scene-2-introduce-absent-macbeth-124809
Well, i have read macbeth many times. There are some parts of macbeth which can help ur question. At first, appearance of dagger in air, due to his imagination, to Mcbeth when he goes to assasin Duncan for the second time because first time, he came back due to his cowardice attitude. Then at the end, when he was being killed, he accused witches for betraying him although they ddnt. He seemed to be a chicken-hearted person at the end who wanted to escape from the charge og a criminal.