I’m reading Macbeth for the first time. The witches prophecize that Macbeth will become king. He and Lady Macbeth immediately jump to the conclusion that this means he has to assassinate Duncan, the current king. Why? If this was me, I’d assume that maybe Duncan will die in a year or two and I’ll become king.
You are quite right that the witches did not prophecy that Macbeth would murder Duncan, and so the option was available to him to “play it safe”: that is, to wait and see how the prophecy transpired, and so, perhaps, ascend the throne of Scotland “holily”.
When you spot a case like this, in which a character behaves in a way that you would never do, that’s a chance to learn something about the difference between the character and you. The question to ask should be, “what do his actions tell us about Macbeth?”
What do we know about Macbeth? He is Thane of Glamis, that is, a member of the nobility of Scotland, who has had to fight in hand-to-hand combat to defend his country from the rebel lord Macdonwald and his Norse allies, the “kerns and gallowglasses from the Western Isles”. How did Macbeth acquit himself in battle? A soldier who witnessed the event reported it to Malcolm thus:
For brave Macbeth (well he deserves that name),
Disdaining Fortune, with his brandish’d steel,
Which smok’d with bloody execution,
Like Valour’s minion, carv’d out his passage,
Till he fac’d the slave [that is, Macdonwald];
Which ne’er shook hands, nor bade farewell to him,
Till he unseam’d him from the nave to the chops,
And fix’d his head upon our battlements.
This does not seem to be a description of a man who would be content to play it safe and wait and see! The soldier says that Macbeth “disdained Fortune”: that is, he risked his life on his skill with the sword. It is clear from this description that Macbeth is accustomed to, and proficient in, bloody violence, and so when he is faced with the question of how to interpret the witches’ prophecy, the dagger is the tool that seems most comfortable and convenient to his hand.
It is indeed true that the prophecy of the three Weyard Sisters does not imply that Macbeth would need to murder king Duncan. In fact, in Holinshed's Chronicles, Shakespeare's source for the play, Macbeth had a genuine claim to the throne, but Shakespeare doesn't really use that in the play. What we have instead is that king Duncan designates his successor and appears to establish primogeniture in Act 1, scene 4:
We will establish our estate upon
Our eldest, Malcolm, whom we name hereafter
Prince of Cumberland
In an aside, Macbeth immediately comments on this as a stumbling block on his way to the throne:
The Prince of Cumberland! That is a step
On which I must fall down, or else o'erleap,
For in my way it lies.
Shakespeare could have developed this obstacle—the prospect of the throne becoming more elusive—into a spur for Macbeth's ambition, but instead of going this psychological route, he portrays Macbeth as a man battling with moral dilemmas.
In Act 1, scene 3, the scene of the prophecy, it is worth looking at Macbeth's reaction to it and how it has been presented on the stage. Rasmussen and Bate note that,
When we, and the weyard sisters, meet Macbeth, he is halfway between the battlefield and home, physically and psychologically unsettled, between two worlds.
Just hours before the prophecy, Macbeth's sword "smoked with bloody execution"; Macbeth and Banquo fought as if "they meant to bathe in reeking wounds" (the captain's description in Act 1, scene 2). So it is not surprising that in 1986, Jonathan Pryce played Macbeth as a "killing machine with an elegant turn of phrase" (interview with Jonathan Pryce, Observer, 9 Nov. 1986, quoted in Bate and Rasmussen). In that production, Macbeth fainted at hearing the prophecy:
His extreme reaction perhaps indicated that he had not just thought about being king, but had already though about killing the king. (Rasmussen and Bate, 140)
Derek Jacobi, who is known for playing more sensitive characters, took a different approach in 1993 but did not overlook the effect that the violence of the battlefield had on Macbeth's mind:
Because of the victory he is in a state of high excitement and of emotional exhaustion. He has been killing all day: he is covered with blood. In this state he gets the news: in this state he must react to it. (Jacobi: "Macbeth", in: Players of Shakespeare 4, quoted in Bate and Rasmussen, 142.)
It is also worth noting, that Macbeth is aware that he may not need to kill king Duncan in order to become king of Scotland. Near the end of Act 1, scene 1, he says in an aside:
If chance will have me king, why chance may crown me
Without my stir.
"Without my stir" means without my needing to take action. So when he does kill Duncan in Act 2, he cannot claim that he could not see another way to become king of Scotland.
- Shakespeare: Macbeth. Edited by Eric Ramussen and Jonathan Bate. The RSC Shakespeare. Macmillan, 2009.
In addition to the two excellent answers by Tsundoku and Gareth Rees, it's worth adding a note about the nature of the play: Macbeth is a tragedy and, in classical theatre, a tragedy is the story of a great person (usually a man) who is brought down and destroyed by a flaw in his otherwise estimable character. Shakespeare's tragedies are classical tragedies, ergo Macbeth must be presented with a choice between doing the right thing (leaving things to fate and ascending to the throne by natural means, if at all), or giving in to the dark side of his nature and seizing the crown by force and bloody murder - if there is no choice, no opportunity to do the right thing, no triumph of the character flaw, there is no real tragedy.
If the prophecy indicated that Macbeth must kill the king, then the tragedy is diminished - there is a malevolent outside force at work. That the prophecy clearly did not require murder to be done puts the onus for what follows directly on Macbeth and his wife.