When do you think Macbeth first dreamt of becoming king? Was it after he talked to Lady Macbeth?
After the prophecies, or might these thoughts have preceeded the witches' prophecies? Explain your interpretation of the line.
In Shakespeare's Macbeth, the title character thinks of killing Duncan to become king at least as early as Act 1.3.136-139, when, in an aside, he refers to his thoughts, thoughts that take the form of a suggestion which is a "horrid image," which "unfix[es} my hair" and makes his "heart knock" at his ribs. The suggestion, Macbeth says, is "Against the use of nature." The suggestion amounts to "horrible imaginings (line 140)."
The suggestion can be no less than that he will need to kill Duncan in order to be king.
The possibility does indeed exist, however, that Macbeth thought not only about being king before the witches make their predictions, but also that he thought about killing Duncan before the predictions.
When the witches hail Macbeth as he who will be king, he starts, as Banquo refers to it. He flinches. Banquo sees Macbeth's reaction and asks him:
Good sir, why do you start and seem to fear
Things that do sound so fair? (Act 1.3.52-53)
Macbeth is not a man to be afraid, and he is not a man to show fear. The witches have given him an exciting title, a glad tiding, if you will. Banquo is right to be surprised. Macbeth, hero of the day's battle, flinches and shows fear when he is greeted as a future king. One has to ask why.
What would make Macbeth flinch? The same horrible imagining that bothers him later. What else? Being king is only fearful, if it takes killing a king to get it. This suggests Macbeth has thought about killing Duncan before, and been filled with horror by the thought before.
Is this a certainty? No. Is it a possibility? Yes.
You do not tell us which line you need to interpret, so I cannot be sure that I am interpreting the line you want explained.
In my opinion, Macbeth does not think of becoming king until after he hears the witches' predictions. In fact, I think he does not really consider it until he is made the Thane of Cawdor. But I do think that he dreams of being king before he talks to his wife.
I base this largely on the part in Act I, Scene 3 where he is made Thane of Cawdor. When this happens, he says that "the greatest is behind." I take this to mean that the best things (the best things that the witches have predicted) are yet to come. This implies to me that he believes now that he might be able to become king.
This is an interesting question to consider, since, in the early scenes of Act One, it is other characters -- the witches, Lady Macbeth -- who confirm that Macbeth will be King. He only repeats the witches prophesies, and then, once he returns home, only answers his wife's wild scheme to kill Duncan with "We will speak further."
However, the very next time Macbeth is onstage (I,vii) is a soliloquy with the audience. Could it be that this is his first real chance to speak his own thoughts, without others putting words in his mouth or on his future?
If it were done when 'tis done, then 'twere well
It were done quickly. If the assassination...
Might be the be-all and the end-all here,
But here, upon this bank and shoal of time,
We'ld jump the life to come.
He doesn't come to any conclusion here, expect that he has
To prick the sides of [his] intent, but only
So, it seems that at least from the mouth of Macbeth himself, this is his first consideration of being ambitious enough to create the circumstance by which to become King.