You need both because they are not quite the same thing, though easily confused. The theme is a general idea that runs through a work and is expressed by it. A motif is a device, a structure, or a literary technique that is used repeatedly to develop and bring out a theme.
One major theme in Macbeth, developed in this passage, is the nature of loyalty and the lengths to which it should be pursued. Macbeth's crime, as he himself has recognized, is fundamentally one of disloyalty. Malcolm, as the acknowledged true king in exile, tests the loyalty of Macduff by suggesting that he, Malcolm, would be an even more evil king than Macbeth has been. By piling vice upon vice, he is able to satisfy himself that Macduff is both loyal and virtuous, because Macduff will not support him if he is indeed a monster of wickedness, but will instead exile himself:
Fare thee well!
These evils thou repeat'st upon thyself
Have banish'd me from Scotland. O my breast,
Thy hope ends here!
In showing that he is loyal both to Malcolm and to virtue, Macduff satisfies Malcolm that he is indeed sincere in his support for the struggle against Macbeth.
Another major theme in Macbeth is that actions have consequences, or more narrowly, that if a person acts rashly they may regret it bitterly later. Macbeth's seizure of power, and his careless trust in the witches, is the main way in which this theme is developed, but it is also illustrated by the fate of Macduff's family. Macduff impulsively and rashly abandons his wife and children to join the rebel cause, and is punished for his impulsive thoughtlessness by their murder at Macbeth's hands.
One motif that appears in this scene, immediately following the conversation between Malcolm and Macduff, is that of politicially motivated violence tearing apart a family. This symbolizes on a smaller scale the violence that Macbeth and his ambitions have done to the Scottish polity. Earlier, Duncan has been killed and his sons forced to flee, and Banquo murdered and his son Fleance on the run. Now, it is Macduff's turn. Macduff, in England flying from the wrath of Macbeth, learns that his family has been murdered at Macbeth's orders:
I cannot but remember such things were,
That were most precious to me. Did heaven look on,
And would not take their part? Sinful Macduff,
They were all struck for thee!
A related motif, that appears tenatively with regard to King Duncan and is asserted strongly at the very end of the play, is the true monarch as a healer who restores the natural order. This motif helps to illustrate the theme of kingship versus tyranny. The King of England, as a true monarch, can heal the sick by his touch:
Their malady convinces
The great assay of art, but at his touch,
Such sanctity hath heaven given his hand,
They presently amend.
Macbeth, on the other hand, though he demands healing for his wife and more broadly for his nation from his court physician, is as an evil king an agent of sickness and ruin.