He knows Macduff has fled to England. After Macbeth left the witches' house, Macbeth decides that Macduff and his family need to be killed because they are a threat to him and will attempt to overthrow him.
So, Macbeth sends killers to kill the Macduff family and take his kingdom.
Macduff kills Macbeth in revenge after Macbeth kills his family. Malcom becomes king because he is the next direct decendent of King Duncan.
Thus, Macbeth sends killers to kill the Macduff family and take his kingdom.
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