To set the stage for act 3, scene 4 of Shakespeare's Macbeth, Macbeth has murdered King Duncan and taken the throne. Duncan's sons, Malcolm and Donalbain, have fled the country, which lessens Macbeth's concerns that they'll challenge his position as King.
Macbeth has ordered the murder of Banquo and Banquo's son, Fleance, in order to thwart the Witches' prophecy that although Banquo wouldn't be King himself, he "shalt get kings" (1.3.70), meaning that he will beget kings, and Banquo already has one son who could replace Macbeth.
Macbeth and Lady Macbeth organize what Lady Macbeth calls a "great feast" (3.1.13), but which Macbeth says is "a solemn supper" (3.1.15), to which they invite Banquo, and Banquo agrees to attend.
Before the banquet begins, Banquo is murdered, but true to his word, Banquo's ghost attends the banquet. This completely unsettles Macbeth. Macbeth already has serious regrets about killing Duncan, and the murder of Banquo compounds his self-recriminations, and nearly drives him mad.
Lady Macbeth does her best to cover for Macbeth's strange behavior at the banquet, but it's clear to all of the guests that Macbeth is not well, and Lady Macbeth insists that everyone leave the banquet as soon as possible.
When Macbeth calms down, he talks with Lady Macbeth, and he tells her that he's going to visit the Witches to find out what's going to happen to him.
MACBETH: I am in blood
Stepp'd in so far that, should I wade no more,
Returning were as tedious as go o'er. (3.4.165–167)
Macbeth has decided, though, that no matter what the Witches tell him—and he expects to hear the worst—that he's going to keep moving forward, and do whatever is necessary to secure the throne for himself. He's caused so many deaths, and he's standing so deep in blood already, that there's no way he can make amends, even if he tried.