Macbeth's killing of King Duncan's guards in act 2, scene 3 leads Macduff to believe that Macbeth is involved in Duncan's murder. In act 2, scene 4, Macduff makes his suspicions about Macbeth known to Ross . Macduff tells Ross that instead of going to Macbeth's coronation, he's...
Macbeth's killing of King Duncan's guards in act 2, scene 3 leads Macduff to believe that Macbeth is involved in Duncan's murder. In act 2, scene 4, Macduff makes his suspicions about Macbeth known to Ross. Macduff tells Ross that instead of going to Macbeth's coronation, he's going back to his own castle at Fife, no doubt to look after his own affairs and to prepare for whatever Macbeth might have planned for him.
In act 3, scene 4, Macbeth reveals that Macduff refused to attend Macbeth's coronation banquet—the banquet which the ghost of Banquo attends uninvited—which angers Macbeth. Macbeth also reveals that he has spies in the homes of the nobility in Scotland, including Macduff's home, so Macbeth is fairly certain that Macduff suspects him of Duncan's murder and that Macduff is likely plotting with other nobles against him.
When Macbeth recovers his senses after being unnerved at seeing the ghost of Banquo at the banquet, Macbeth tells Lady Macbeth that he's going to the witches to find out what they know about his future.
In act 4, scene 1, an apparition conjured up by the witches tells Macbeth to "beware Macduff; / Beware the Thane of Fife" (lines 79–80). Macbeth already has concerns about Macduff, but before Macbeth can question the apparition further about Macduff, it disappears. A second apparition appears and tells him "none of woman born / Shall harm Macbeth" (lines 90–91), which Macbeth interprets to mean that no one, including Macduff, can harm him or imperil his throne.
Nevertheless, Macbeth vows to kill Macduff to "make assurance double sure" (line 92), if for no other reason than Macbeth can sleep better at night—"and sleep in spite of thunder" (line 96)—knowing that Macduff is dead and that he's no further threat to him.
Just a little later in that scene, however, Lennox tells Macbeth that "Macduff is fled to England" (line 158), which further angers Macbeth because he missed the opportunity to kill Macduff in Scotland, and now Macduff is out of reach in England. Macbeth resolves not to hesitate when he decides something must be done but to do it immediately. This leads Macbeth to his fateful decision to attack Macduff's castle, to "give to the edge o' the sword / His wife, his babes, and all unfortunate souls / That trace him in his line" (lines 168–170), before any more time passes and "before this purpose cool" (line 171).
Macbeth immediately sends murderers to Macduff's castle to do what Macbeth said he intended to do, which was to kill Macduff's wife and all of his children.
Consistent with Macbeth's firmness of purpose and his intent to attack Macduff's castle without delay, Shakespeare wastes no time in presenting that horrific scene to the audience in act 4, scene 2.