Aethelred II was born about 968AD, son of King Edgar the Peaceful and his second wife Aelfthryth (otherwise Elfrida). His name meant "Well-Counselled". He was crowned in 979, at about the age of 10, after his elder brother Edward the Martyr had an accident at Corfe Castle (he accepted a drink of mead from his step-mother, and one of her men stabbed him, in the back). Ethelred witnessed this and cried inconsolably until his mother, Aethelthryth, beat him with candles, after which he could never bear the sight of candles in his presence. What he did when it got dark is attested by chroniclers who said that he slept a lot and in the remarkable number of children he fathered. Much of his later actions can be viewed in the context of borderline personality disorder as a result of traumatic childhood experience. Coming to the throne aged 10, he was dominated by the party opposed to the Benedictine faction of Dunstan, Archbishop of Canterbury and was literally cursed by the circumstances in which he came to the throne. Even before this Dunstan is said to have cursed him when he allegedly defecated in the font whilst being christened. On crowning him, Dunstan gave a speech slagging him off.
The King could either "pay or play" -- he could either bribe them to go away or he could fight them. The legacy of his father, King Edgar's reign under the stewardship of St.Dunstan, gave him substantial resources squandered on paying off his enemies, rather than rewarding supporters or building armed forces. Unfortunately even when he did, he could not rely on the loyalty of his commanders (such as Wulfnoth Cild or Aethelric of Mercia). He also lacked for counsellors with, frankly, spine, to advocate one course or the other through thick and thin. Thus he gained his later byname of Unraed -- the Uncounselled, or the Ill-Counselled (perhaps also became of his selection of Eadric Streona as a son-in-law and chief counsellor -- a man later to be demonised as a multiple traitor).
Eadric was useful to Ethelred in arranging the assassinations various potential dissidents such as Aelfhelm of York and Morcar and Siferth and allegedly his son and rival, Edmund Ironsides. He also took the vital position of Ealdorman of Mercia, which had been left vacant after the treason of Aethelric (which Ethelred punished by seizing his son, Aelggar, and burning his eyes out with white-hot pokers.
The Millenium seems to have spurred him into action -- he found an army and a fleet, and went to war against Malcolm, his Cumberland under-king, who had been remiss in contributing to the danegeld taxes. Two years later he sorted out a dispute with the Duke of Normandy by marrying the duke's sister Emma, the Pearl of Normandy. Their first son, born within the year, was to be Edward the Confessor.
It was the same year (1002)Aethelred decided to solve the Viking problem by having all Danes living in England killed in the St.Brice's Day Massacre. However this outrage did not affect the fighting forces of the Isle of Wight and in the Danelaw, except to enrage them, especially after the murder of Gunnhild, the sister of Sweyn Forkbeard, the Danish King. It is possible that the intended victims were Danish mercenaries (referred to by Palgrave as "stipendiary bravos")but popular hatred of the Danes allowed it to degenerate into a mass pogrom. In 1006 the Vikings returned; the next year Aethelred bought them off; in 1009 they returned. Ethelred raised a huge fleet at Sandwich, but Brihthelm, brother of Eadric Streona accused his nephew, Wulfnoth Cild of Sussex of treachery. Wulfnoth made off with a third of the fleet, followed by his vengeful uncle, who ran into a storm. The local ships of Wulfnoth returned and burned the wrecked fleet, before deserting to the Danes in East Anglia. In 1012 they martyred St AlfheahArchbishop of Canterbury at Greenwich, outside London. In 1013 they invaded in force and Aethelred fled for his life to his brother-in-law, Robert of Normandy, leaving England to Sweyn Haraldsson ('Forkbeard'). He died in 1014 and Aethelred returned, to reclaim the throne, and died in 1016, being succeeded by his son Edmund.
His 37-year reign was, in fact, one of the longest in English history, and introduced several long-standing features including the introduction of the 12-man jury.
Disclaimer: Parts of the above have been inspired by a re-reading of Cyril Hart's Between Severn (Saefren) and Wye (Waege) in the Year 1000.