Born in 1001, Duncan was the son of Crinan, Abbot of Dunkeld and ruler of Atholl, and Bethoc, daughter of Malcolm II. The son of an abbot, you ask? Some scholars infer that the abbacy had become a secular position at this point; others point out that the Celtic Christian church did not require its clergy to be celibate, and they see a married abbot as a holdover from earlier times. I like this second explanation for two reasons: one, there are documented instances in England during this time period of clergy having "secular" marriages that were not recognized by the Church; and two, when Macbeth's successor married Margaret of England, she became known for her prodigious efforts to stamp out non-Roman Catholic practices in Scotland.
At 17--"legal age" among the Scots at that time--Duncan became king of Cumbria, a client-kingdom of Alba. Some scholars infer that this made him Malcolm II's tanist, or designated heir, as Cumbria seems to have been used as a training ground for future Alban kings. If that is true, it would undoubtedly have angered the house of Kenneth mac Dubh, who expected one of theirs would succeed Malcolm.
If that weren't grievance enough, the annals record that in 1033 the son/grandson of Boite was killed by Malcolm. We do not know if this was a death in battle, or if Malcolm simply had this scion of the house of Kenneth mac Dubh assassinated. At any rate, his outraged relatives--kin to Macbeth's wife--rose up against Malcolm II. They were defeated in battle, but Malcolm received his death wound and perished at Glamis in 1034.
As the designated choice of a strong and apparently popular king, Duncan became the ruler in Alba. But shortly thereafter, he led an ill-fated attempt to take the city of Durham in Northumbria, some distance across the English border. In a strategic blunder, he sent his cavalry in first against the walled city; they were devastated by the Northumbrian archers, and when next Duncan's foot soldiers charged the walls, the Northumbrian cavalry rushed out from the city and cut them down. 3000 Scots died. The Northumbrian women gathered their heads, washed them and braided their hair, and they were put on pikes around the city walls.
Duncan's constituency cannot have been pleased with their new king.