I assume you are refering to the end of Act III scene 4, after Macbeth has organised for the murder of Banquo and has seen his ghost dramatically during a banquet in front of his Lords. At this point, he tells his wife that he intends to visit the witches again. He himself interprets the importance of this action for us, so pay particular attention as to how he explains the reason for wanting to make this visit:
I will tomorrow
(And betimes I will) to the Weird Sisters:
More shall they speak; for now I am bent to know,
By the worst means, the worst. For mine own good,
All causes shall give way: I am in blood
Stepp'd so far, that, should I wade no more,
Returning were as tedious as go o'er.
Macbeth thus thinks that he has done so many bad things now that he might as well carry on. We have seen his gradual progression from being cajoled to committing murder to cold-heartedly organising the murder of a former friend himself, and now he is, as he says, "bent to know... the worst." He has given himself over to evil, and recognises that the witches are the representatives of evil on his plane, and thus seeks to know the rest of how the story will turn out.