Quote: 'It will have blood, they say; blood will have blood'
Banquo's ghost suggests he's seeking revenge on his death, and Macbeth notices this.
Now that Macbeth has taken life, his life will be taken- his bloody path of murder must lead to other murders
Audience may be fearful of what will happen to him because of his actions
Macbeth alludes to the sons of Duncan earlier in Act 3, scene 1, with the word "bloody"
the Elizabeth Chain of Being: whatever affects one thing affect others
Alternative interpretation: the shedding of Banquo's blood will have to lead to (get) the shedding of his sons' blood as well, because the prediction was that Banquo should be the father of kings, which Macbeth can't allow.
Reference to the predictions of the three witches in Act 1, scene 3, who have told Banquo that his "blood" will have "blood" (sons)--"Thou shall get kings, though thou be none"
Macbeth is feeling both guilt and paranoia.