The final scene of Act I of Macbeth opens with the title character vacillating over whether to carry out the murder of Duncan, who is about to retire to his chamber after the banquet at Macbeth's castle. He is concerned with the implications of the act, which he fears will bring negative consequences for the present and the future:
But in these cases
We still have judgement here, that we but teach
Bloody instructions, which being taught return
To plague the inventor. This even-handed justice
Commends the ingredients of our poison'd chalice
To our own lips.
Macbeth acknowledges that he has no real quarrel with the king, but is driven only by "vaulting ambition" to kill a man who is his lawful monarch, his kinsman, and his guest. By the end of the soliloquy, it seems that he has determined not to murder Duncan, but Lady Macbeth arrives, and, challenging his masculinity and his courage, goads him into the deed. This exchange raises important questions over who is more responsible for the chain of events that follow.