In Shakespeare's Macbeth, the man we see at the start of the play as he plans to kill Duncan is different than the man he is in Act Two, scene two. The struggle he goes through to come to terms with his actions will define how easily he is able to commit murder at the end of the play.
Up until this point, Macbeth has expressed some misgivings about the plan that he and his wife are "hatching" to kill Duncan while the King is staying with them. However, while Macbeth's concerns are more based upon how these actions now come at an awkward time (as things are going so well), Macbeth resigns himself to carrying out the task at his wife's insistence without much resistance overall. In scene one of the second act, the dagger scene might make the audience believe that Macbeth would be fearful in seeing the apparition of the dagger that directs his path—even while showing him the blood and gore on its blade that will soon appear after Macbeth completes the task.
By the time Macbeth is finished killing the King, this soldier and honored warrior is coming apart at the seams. He is hysterical and struggling to deal with what he has done. The cool mind that was in charge in the previous scene is gone, and is replaced with someone who is falling apart—as if he has never seen death before; perhaps this is true in the sense that he has never seen the murder of a King, especially at his hands, before.
This parallels the way Macbeth changes as he moves through the play. At first, he is hesitant, but then he throws himself into his new plan to become King. We can identify this as foreshadowing Macbeth's future actions: when he will finally do anything to protect his place on the throne. As he comes to terms with what he has done, killing becomes much easier. He admits that he only needs "practice" to get better at it. By the end of the play, Macbeth's sense of honor has completely dissolved. What we see at the beginning of the play is a clear indicator that while a young murderer he is frightened, time does make the act much easier for him.