THIRD WITCH. All hail, Macbeth, that shalt be King hereafter! (1.3.53)
Macbeth kills Duncan to become King.
Macbeth regrets killing Duncan but believes that it was necessary to fulfill his destiny. Nothing personal. Just business. For the moment, Macbeth feels confident and secure in his place as King.
THIRD WITCH. (to Banquo) Thou shalt get kings, though thou be none. (1.3.70)
Macbeth orders the death of Banquo and his son, Fleance, to stay King. He is mindful of the witches's prophecy that Banquo won't become King himself but that Banquo's descendants will become kings.
There's no immediate threat to Macbeth from Banquo or Fleance, but Macbeth is becoming fearful and paranoid. He wants to avoid any possibility that Fleance will become King or that Banquo will have any more defendants, so he orders that both of them be murdered.
MACBETH: To be thus is nothing,
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be fear'd. ...
There is none but he
Whose being I do fear...
He chid the sisters,
When first they put the name of King upon me,
And bade them speak to him; then prophet-like
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown...
For Banquo's issue have I filed my mind,
For them the gracious Duncan have I murdered...
To make them kings, the seed of Banquo kings! (3.1.52-74)
The murders of Duncan and Banquo (Fleance escapes) are purposeful, necessary (to Macbeth), individual, and impersonal.
THIRD APPARITION. Beware Macduff! (4.1.78-79)
Macbeth responds that he was thinking the same thing.
Another apparition tells Macbeth that "none of woman born / Shall harm Macbeth" (4.1.78-79, 89-90). Macbeth decides that Macduff can't do him any harm but decides to have him killed anyway to assuage his growing fears of being overthrown as King.
MACBETH. Then live, Macduff. What need I fear of thee?
But yet I'll make assurance double sure,
And take a bond of fate: thou shalt not live,
That I may tell pale-hearted fear it lies,
And sleep in spite of thunder. (4.1.92-96)
Macbeth is told by Lennox immediately after the scene with the apparitions that Macduff has escaped to England. Macbeth knows that Macduff has gone to England to meet Duncan's son, Malcolm, in order to restore Malcolm to his rightful place on the throne of Scotland.
Macbeth is upset that he missed the opportunity to kill Macduff while Macduff was still in Scotland, but he decides to seize Macduff's lands and property while Macduff is in England and to teach him a further lesson.
MACBETH. The castle of Macduff I will surprise,
Seize upon Fife; give to the edge o’ the sword
His wife, his babes, and all unfortunate souls
That trace him in his line. (4.1.167-170)
By now, Macbeth is not only fearful and paranoid, but he's also becoming increasingly desperate to keep his throne.
The murder of Macduff's wife, his children, and his household servants is personal, vindictive, purposeless (except to appease Macbeth's vengefulness against Macduff), and ultimately self-destructive.
One of Macduff's prime motivations for returning to England and facing Macbeth in battle is to avenge the death of his wife and children.
MACDUFF: ... Tyrant, show thy face!
If thou beest slain and with no stroke of mine,
My wife and children's ghosts will haunt me still. (5.7.18-20)