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Macbeth:
Your spirits shine through you. Within this hour at most
I will advise you where to pant yourselves,
Acquaint you with the perfect spy o' th' time,
The moment on 't; for 't must be done tonight,
And something from the palace; always thought
That I require a clearness: and with him-
To leave no rubs nor botches in the work-
Fleance his son, that keeps him company,
Whose absence is no less material to me
Than is his father's, must embrace the fate
Of the dark. Resolve yourselves apart:
I'll come to you anon.
3.2.128-139

Macbeth realizes that Banquo is the one that was there with him and knows the prophecies. Basically, Banquo knew too much and Macbeth wanted him killed. He hires three murderers to kill Banquo as well as his son, Fleance, but Fleance escapes. Macbeth is starting to feel paranoid by the witches' prophecies and takes as many precautions as he can including killing whoever is a threat to his life. To become king he murdered Duncan in the night with a well thought out plan with Lady Macbeth, he feels terrible guilt throughout the play. Now to insure his safety he takes to drastic measures.