In Macbeth, why does Macbeth call Malcolm a "step on which" he "must fall down or else o'er leap"?Which of these alternatives do you expect Macbeth to choose and how might he do it?
In Shakespeare's Macbeth, the lines you ask about are from Act 1.5, while Duncan is king of Scotland. These lines are spoken before Macbeth kills King Duncan.
Duncan, in fact, has just named Malcolm Earl of Cumberland, which is the same as naming Malcolm his heir. The line of succession for the throne of Scotland at this point, should something happen to Duncan, the present king, is Malcolm, Duncan's eldest son, Donaldbain, Duncan's other son, and, then, apparently, Macbeth, as the most celebrated warrior in Scotland.
The witches have predicted that Macbeth will be king. If this is to happen soon, then Macbeth will have to step over, figuratively, Malcolm, since he is Duncan's heir. Macbeth is speaking metaphorically. Malcolm is compared to an obstacle that Macbeth will have to step over in order to be king. The obstacle will either stop Macbeth, or Macbeth can leap over it and achieve his goal--the throne of Scotland.
No deep or convoluted interpretation is needed here. Macbeth simply uses a metaphor to reveal Malcolm as an obstacle to his achieving the throne. It's just a simple metaphor, used by Macbeth before he is king to reveal what he will have to do to become king, in addition to killing King Duncan. The obstacle, figuratively, ends up removing itself, when Malcolm and Donaldbain both flee Scotland after their father is killed. Their flight leaves Macbeth next in line for the throne.
I agree. Macbeth feels that Malcolm stands in his way to the throne. It must be remembered that at this time, that father to son succession was not necessarily the way kings were chosen. We are dealing with Middle Ages Scotland. The belief in the Divine Right of Kings was a Renaissance idea. In this more warlike period where Norse raids were common, the king needed to be a strong warrior. Macbeth has already proven that he is a capable leader. Malcolm, on the other hand, has not. He needed to be rescued. Besides, the weird sisters have predicted that he will be king. When Duncan names his successor, Macbeth has every right to believe that he is the most able successor and should be named king which is why he is surprised at Duncan's choice. If the succession were father to son, Duncan would not have to name Malcolm since it would be obvious. Macbeth has every right to believe that he will be the next king. Didn't he just save Malcom's life?
Macbeth clearly understands that Malcolm is the rightful heir to Duncan's throne, and Macbeth was only given the title of King because Malcolm and Donalbain fled in fear for their lives. However, if Malcolm returns to Scotland, Macbeth will be removed from his position as King. So, Macbeth says that Malcolm is either a step on which he must fall (meaning that he must give up the throne to Malcolm) or a step which he must overleap (meaning that he must prevent Malcolm from returning and claiming the throne). Malcolm is an obstacle in Macbeth's plan to continually rule Scotland, so Macbeth must find a way to deal with Malcolm. Macbeth is more likely going to choose getting rid of Malcolm similar to how he got rid of King Duncan. Macbeth must keep any suspicion away from himself so that he is trusted by the people, so he would likely hire someone to kill Malcolm like he did to kill Banquo.
I would like to add certain things to the already given answer by concentrating first on the image Macbeth uses over here in act I scene 4, while he is in Duncan's court. This is after the temptation of the witches where they did not specify the way Macbeth can become the king of Scotland. They only declared the future that he will be so. Macbeth at the end of that scene had decided that if fortune wants to confer this title on him, let her do it absolutely on her own. Macbeth would not take an active step in trying to become the king.
In the present scene, when Duncan declares his son Malcolm the prince of Cumberland, it is clear that he will be the next king and not Macbeth. This declaration about Malcolm thus makes it necessary for Macbeth to take an active step if he wants to become the king. This image of 'overleaping' relates to the Renaissance image of man as an 'overreacher' always trying to outdo himself, asking for more and bringing the house down on himself. This image also corresponds with the image of 'vaulting ambition' in the play.