For plot purposes, Macbeth has to become a terrible tyrant. There is no psychological reason why he should become so tyrannical just because he killed Duncan, or because he had Banquo killed, or because he had dealings with the witches. He could have become a good king and capable ruler, which was probably exactly what he intended. He certainly seems to have those benign capabilities when the play opens. Claudius in Hamlet is a villain and a usurper, but he seems to be a wise and popular monarch. The reason Shakespeare had to emphasize Macbeth's tyranny was to make it plausible that the English king, at great expense, would authorize the raising of what in those days was a enormous army of ten-thousand men to invade Scotland. Otherwise, the English king would not have felt morally justified in interfering in Scottish politics.
Malcolm and Macduff are not this king's friends or favorites. Macbeth has been lawfully elected king of Scotland, and it would be impossible for Malcolm to ask the English king for military assistance just because Malcolm was next in the line of succession. The English king has no way of knowing that Malcolm was not responsible for Duncan's murder. Even if the king had believed Malcolm's story, those assassinations and coups were common in Scotland and none of his business. He had nothing to gain by trying to place Malcolm on the Scottish throne, and of course there was no assurance that his army would have been victorious. He could be making a bitter enemy of Macbeth and even risking a counter-invasion.
Another reason why, for plot purposes, Macbeth had to be depicted and reported as tyrannical was to make it plausible that so many people would flee Scotland to join the ranks of the English army. Evidently, those who were fleeing were not only many of the important thanes, but liegemen of those thanes and ordinary commoners as well. This flight of so many people was creating additional havoc in Scotland. Crops were being neglected and animals untended. A state of anarchy was emerging. If economic hardships developed in that country, it could affect English imports and exports and might also lead to a influx of refugees who could commit thefts, robberies, burglaries, assaults, and murders, and who could become a permanent threat and liability to the English people and their government.
Thus, the unprecedented crisis in Scotland would give the English king a practical motive for attempting to overthrow the great tyrant Macbeth, and Macbeth's tyranny would lead to the swelling of the ranks of the English army, which could make the difference in the military balance. Macbeth himself says in Act V, Scene 5:
Were they not forced with those that should be ours,
We might have met them dareful, beard to beard,
And beat them backward home.