In Act I, Scene 3 I think that the main idea is that there is a debate between Macbeth and himself. Lines 128 - 142 talk about Macbeth's new situation of meeting with three witches, who prophesise that Macbeth will become Thane of Cawdor, and then he will become king. This is made true when Macbeth becomes the Thane of Cawdor, and now Macbeth is thinking about becoming king and what it would take. He ponders to himself that maybe he might have to provoke the prophecy so it would come true, as in take measures to make sure he becomes king( ie. Killing Duncan). Then it is thought by Macbeth because he has become Thane of Cawdor that he will and must become king and when he doesn't think that may happen naturally he contemplates the thought of killing Duncan. .
Lines 128-142 also show that the mind of Macbeth is thinking about the idea of murder. It makes him think that, what if he hadn't met the witches, would he still have become Thane of Cawdor, but because the witched did exist and the first prophecy came true that the second one will and he has to help.
The witches have a large impact on Macbeth killing the king. This is so because if they hadn't have prophecised this, then Macbeth wouldn't have even had the idea of killing the king. I think this gives them both an evil picture. The witches for one for even telling him that he would become king because this sparked the thought to kill Duncan, and they would've known that Macbeth would end up killing him if they knew he was to be king. Also Macbeth gets an evil picture portrayed because he actually was trusted by the king and now he goes and backstabs him and kills him when Macbeth was a warrior/noble for the king. .