How does Macbeth react when Duncan declares his son Malcolm heir to the Scottish throne?
In Act I scene iv, Macbeth (now also named the Thane of Cawdor) is starting to believe the witches earlier prediction. The witches predicted that he would be Thane of Glamis, Thane of Cawdor, and a king of Scotland. Since two of the three have come true, he believes it's a possibility that he could be king. However, when King Duncan names Malcolm as his successor, Macbeth becomes upset. He says to himself:
The Prince of Cumberland! That is a step
On which I must fall down or else o’erleap
For in my way it lies. Stars, hide your fires;
Let no light see my black and deep desires.
He knows that if he is going to become king, he is going to have to step over Malcolm or give up. In order to step over him, he knows he is going to have to kill him. However, he also knows he has to hide these desires until the appropriate time.
In the most basic terms, Macbeth reacts with shock at Duncan's announcement of Malcolm as heir. The Scottish system of ascendancy to the throne was different from England's system of primogeniture (first born son takes the throne). Scotland's system was that of tannistry whereby ascension to a vacated throne was the possible right of any male heir, which, of course, led to considerable blood shed. In effect, the law of tannistry meant that the last heir standing was the new king. Duncan was instituting ascension by rule of primogeniture instead, even though he was fully aware of and appreciative of Macbeth's superior qualities: Duncan recognized a higher good in initiating rule by a bloodless law. Ironically, perhaps Macbeth's ascension under the law of tannistry may have been bloodless because it seems all the other thanes supported him. However, Duncan had one chance and one chance only to initiate a peaceful law of ascendancy in Scotland.
All this eluded Macbeth. He was only shocked and apparently enraged by Duncan's words. Macbeth didn't hear or understand the praise and honor he was being bestowed. He didn't see that Duncan had a greater good in mind for Scotland than the selection of one new king. Again ironically, perhaps it was Macbeth's superior qualities that convinced Duncan it would be safe to institute primogeniture in place of tannistry because he believed he could rely on Macbeth to support him and to see the advantage to the thanes and Scotland. However, Macbeth's ambition and shock led him to embrace the spirit of darkness to conceal his foul plans and deeds and to shield his own eyes from his deed. Since Macbeth consults witches, his primary concern may not have been to evade the eyes of God in the darkness but rather to shield his own sight from his deeds, thus sparing himself some of the horror of his own actions.
For more information on Macbeth's reaction to Duncan's announcement, please read the excellent article written by the San Ramon Campus of Diablo Valley College English 154 Class the content of which is taken from audiotaped lectures about Macbeth prepared and delivered by William Harlan Ph.D.
At the end of act I scene III or the so-called temptation scene, Macbeth had contemplated Duncan's murder as a horrid image that unfixed his hair. But, he had left it on his destiny to crown him with the royal title. He had decided against taking any active step to get to the throne. But it is Duncan's announcement of his son Malcolm as the Prince of Cumberland, that seals Macbeth's fate. Now he sets up Duncan's murder as the only way to get to the throne. Thus we see a shift in his predicament from a passive predeterminism to an individualist agency. His aside uncovers his secret self-communication. "Stars, hide..." speech is an invocation to the evil powers of the dark to help him avoid moral justice and the eye of God. This is what shows us that the ethical consciousness is still operative in Macbeth. The speech can be seen as a parallel to Lady Macbeth's invocation to darkness that follows up. The integrity of the earth is another point he makes in the speech. The firm-set earth is not supposed to reveal his murderous strides and moves, disclosing his whereabouts.