After the short appearance from the witches, Macbeth has started to genuinely think about his future. At this point he has started to realise that in order to become King of Scotland, he must overcome the obstacles standing in his way. At the moment, the most apparent obstacle is Malcolm (The Prince of Cumberland). .
In this passage the reader first discovers that Macbeth is quite ambitious and has a relatively strong desire for the throne. Although later on in the play, we find that his desire wavers slightly as his wife pressures him as he ponders to intently on the consequences for various actions. We also learn that Macbeth is slightly gullible to have taken the witches so seriously and to have been planning so early for his conquest over the throne and it's heir. * Also is seems that Macbeth wants to get the crime over and done with but he does not want to become accustomed to the deed so that he will not become a murderous man.
MacBeth's realisation of obstacles is evident when he says that the Prince of Cumberland is a step on which he must fall over or otherwise over leap. We know that he has dark intensions and is desperate for the crown when he tells the stars to hide their fires so that his evil plans would not be discovered. .
I think that Macbeth does not want to become a murderous man because he says "let the eye blind itself to what the hand does but let it be what the eye fears when it is done".