On the death of the Mormaor she married Macbeth, and when Macbeth slew Duncan (1040), he was removing a usurper--as he understood it--and he ruled in the name of his stepson, Lulach.

The power of Duncan had been weakened by repeated defeats at the hands of the Northmen under Thorfinn.

In 1057 Macbeth was slain in battle at Lumphanan, in Aberdeenshire, and Malcolm Canmore, son of Duncan, after returning from England, whither he had fled from Macbeth, succeeded to the throne.

But he and his descendants for long were opposed by the House of Murray, descendants of Lulach, who himself had died in 1058.
The world will always believe Shakespeare's version of these events, and suppose the gracious Duncan to have been a venerable old man, and Macbeth an ambitious Thane, with a bloodthirsty wife, he himself being urged on by the predictions of witches.

He was, in fact, Mormaor of Murray, and upheld the claims of his stepson Lulach, who was son of a daughter of the wrongfully extruded House of Aodh.
Malcolm Canmore, Duncan's grandson, on the other hand, represented the European custom of direct lineal succession against the ancient Scots' mode..