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In a simple sentence, because he gets the crown and his guilt disappears. Straight after the murder, Macbeth feels extremely guilty, refusing to go back to leave the daggers with Duncan's grooms, and succumbing to horrible imaginings about his bloody hand reddening the sea:
Will all great Neptune's ocean wash this blood
Clean from my hand? No, this my hand will rather
The multitudinous seas incarnadine,
Making the green one red.
There's then an uneasy performance the next morning when Macbeth tries to pretend he knows nothing about the murder. And then the next time we see him, he's crowned. And he has to try and secure his throne - remember that Banquo's children are predicted to be kings!
So, to secure his throne he tries to kill Banquo (and his son Fleance, who escaped) and then to weaken Macduff, has Lady Macduff and her children murdered (mainly because the witches tell him to "Beware Macduff").
Macbeth keeps killing. But he does feel guilty - and he isn't sleeping, as the following soliloquy explains:
But let the frame of things disjoint, both the worlds suffer,
Ere we will eat our meal in fear and sleep
In the affliction of these terrible dreams
That shake us nightly. Better be with the dead,
Whom we, to gain our peace, have sent to peace,
Than on the torture of the mind to lie
In restless ecstasy.
He is guilty, but he keeps killing. One of the lessons of the play is that holding a moral opinion doesn't have to affect your actions.
Macbeth keeps killing because he develops a sense of paranoia after he is crowned king. He will do anything necessary to protect his throne. After he kills Duncan and becomes King, Macbeth slowly starts to lose his sense of morality, the same morality that allowed him to be a brave and loyal subject of King Duncan.
He kills Banquo because he is a threat. He wants to kill Macduff, but finds that he is not home, his murderers kill all the members of his household instead.
Macbeth's decision to keep killing is also influenced by the witches prophecy. Macbeth is fooled into believing that he can control his destiny and protect his future. What he doesn't know is that the witches are using him to exact his own ruin.
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