In Macbeth, Shakespeare seems to be showing us that the effort required to attain power, no matter how difficult attaining that power might be, is significantly less than the effort required to keep that power.
Within minutes of killing Duncan, Macbeth faces the first of many challenges to the power he believes he's on his way to acquiring.
In the excitement of watching Macbeth accede to Lady Macbeth's urgings and kill Duncan, the audience assumes that when Macbeth kills Duncan, Macbeth becomes king. The audience forgets that killing Duncan is just the first step on Macbeth's way to becoming king.
In act 1, scene 4, after the battle in which Macbeth fought so bravely, Duncan gathers his nobles together to tell them that he's naming his son, Malcolm, as heir to his throne. Even while he's contemplating killing Duncan early in the play, Macbeth knows that he'll have to do more than kill Duncan to become King.
MACBETH: The Prince of Cumberland! [Malcolm]. That is a stepOn which I must fall down, or else o'erleap,For in my way it lies (1.4.55–57).
After Duncan is found dead by Macduff, Macbeth and Lady Macbeth have to keep suspicion for Duncan's death from falling on Macbeth. It's not until Malcolm and Donalbain walk into the scene that the audience is reminded that Duncan's sons stand in the way of Macbeth acquiring the throne.
However, Malcolm and Donalbain make it easy for Macbeth by fleeing the country, hoping to avoid their father's fate, and Macbeth assumes the throne in Malcolm's absence.
An added benefit to Macbeth is that by leaving the country so soon after Duncan's death, Malcolm and Donalbain have brought suspicion for Duncan's death on themselves and away from Macbeth.
MACDUFF: Malcolm and Donalbain, the King's two sons,Are stol'n away and fled, which puts upon themSuspicion of the deed (2.4.33–35).
Macbeth realizes that simply becoming king isn't enough. He has to stay king.
MACBETH: To be thus is nothing,But to be safely thus (3.1.52–53).
Macbeth hasn't forgotten the Witches' prophecy to Banquo that although Banquo would not be king himself, his descendants would become kings.
MACBETH: Our fears in BanquoStick deep . . . There is none but heWhose being I do fear . . . He chid the sisters,When first they put the name of King upon me,And bade them speak to him; then prophet-likeThey hail'd him father to a line of kings:Upon my head they placed a fruitless crown (65)And put a barren sceptre in my gripe,Thence to be wrench'd with an unlineal hand,No son of mine succeeding (3.1.53–68).
Macbeth arranges for murderers to kill Banquo and his son, Fleance, to rid himself of anyone in Banquo's line who might rise up against him.
Macbeth confides his fears to Lady Macbeth.
MACBETH: O, full of scorpions is my mind, dear wife!Thou know'st that Banquo and his Fleance lives (3.2.40–41).
Macbeth is happy to learn that Banquo has been killed, but Fleance escaped.
MACBETH: Then comes my fit again: I had else been perfect,Whole as the marble, founded as the rock,As broad and general as the casing air:But now I am cabin'd, cribb'd, confined, bound inTo saucy doubts and fears (3.4.23–27).
Macbeth rationalizes that for now, Fleance is not a threat, but the thought of Fleance challenging him for the throne is a constant reminder of how insecure his throne really is.
Soon, another, more pressing threat to his throne arises. Macduff is raising an army against Macbeth. Macbeth goes to the Witches and demands answers to his questions.
The Witches conjure up apparitions, and the first thing that the apparitions tell Macbeth is, "Beware Macduff" (4.1.78–79).
Macbeth is well aware that Macduff is to be feared, but he puts that warning aside when other apparitions tell him that "none of woman born" can harm him, and that he "shall never vanquish’d be until / Great Birnam Wood to high Dunsinane Hill / Shall come against him" (22.214.171.124–105).
Macbeth is in fear for his throne and in fear of his life. In his desperation, he hears what he wants to hear. He ignores the most important admonition to "beware Macduff" and relies instead on the false hope given to him by the the other apparitions.
The power that Macbeth usurped from Duncan has not allowed him to enjoy even a moment's peace. He's had to fight continually to keep his power until he paid the ultimate price for his treachery at the hand of one who was "none of woman born."