Malcolm asks Macbeth why he left his family alone in Scotland to come to England.
Malcolm fled to England after his father King Duncan’s death. His father had named him successor to the throne, and he knew he would be a target even if he was not suspected of murdering the king.
Macduff comes to see Malcolm, and Malcolm tries to tempt Duncan into thinking he is unfit as a king. He makes several suggestions about his lack of preparation and various character flaws. He even comments about Macduff leaving his family at Macbeth’s mercy.
Why in that rawness left you wife and child,(30)
Those precious motives, those strong knots of love,
Without leave-taking? (Act 4, Scene 3)
Malcolm did not know that Macbeth’s family was going to be murdered. Macbeth was concerned about Macbeth knowing too much about Duncan’s murder, and about Macduff’s fierce loyalty. Malcolm was aware that Macbeth would stop at nothing to get his way.
Macbeth had Macduff's wife and son killed, and the entire family staff. Malcolm urged Macduff to turn his grief into rage, and fight back.