Macbeth tells the murderers that he has his own reasons for
wanting Banquo murdered, and he provides them with several reasons
why they should do it before he challenges their masculinity (a
quick study--he's learned from the master--oops mistress).
We aren't told precisely what Macbeth has done, but that he has
done something can be inferred from his comments to the murderers,
all of whom apparently have a grudge not against Banquo, but
against Macbeth. Macbeth tells them that it was Banquo and not his
"innocent self" that caused their undoing. In fact, he tells
them that he has already informed them of exactly how and by what
means the offensive deeds were done. According to Macbeth, Banquo
is the one whose "heavy hand hath bowed [them] to the grave and
beggared them forever" (by which we understand that Macbeth must
have destroyed their lives and their livelihood so completely that
they are all desperate men.