Are you referring to the 9th or 11th century "conquest?"
Okay, so here we go.
The story begins with Æþelræd Unræd. Latterly known as "Æthelred the Unready," though the transcription and meaning of the words has changed so much that the pun isn't apparent. The original Anglo-Saxon comes out something like Æþelræd "wise council," Unræd "the ill-counselled." So the meaning has changed from "badly advised" to "unprepared." He ascended to the throne under something of a cloud, because his brother was murdered, though Æthelred was a child at the time and seems to have escaped most of the blame. Who were there advisors and what was this bad advice? Well, the Anglo-Saxon Chronicles has a hint. It's about this time that the Vikings are starting to get out of hand: the north of England has always been a cultural hodgepodge of Norsemen, Angles, and Saxons, and the Danish raiding fleets seem to be receiving little resistance from the north of England. So Æthelred has an idea:
A.D. 1002. ... and also, in the same year the king gave an order to slay all the Danes that were in England. This was accordingly done on the mass-day of St. Brice; because it was told the king, that they would beshrew him of his life, and afterwards all his council, and then have his kingdom without any resistance.
A.D. 1006. ... Wulfgeat was deprived of all his property; Wulfeah and Ufgeat were deprived of sight; Alderman Elfelm was slain
A.D. 1007. ... In this year also was Edric appointed alderman over all the kingdom of the Mercians.^1
So: straight out the gate Æthelred goes with "genocide" as an answer to the problem of Danish settlers having incorporated themselves into the north and lending assistance to Viking raids. Everyone important in the north of England is getting daggers in the back, and Edric is given a choice appointment in the north and is married to a sister of Æthelred. Of course, the people in England aren't stupid, and for a period of two years King Æthelred is driven into exile by his own people and they choose Sweyn Forkbeard, Viking invader, as king of (Northern) England. Because at least he isn't conspiring to murder you. But then:
A.D. 1014. This year King Sweyne ended his days at Candlemas, the third day before the nones of February; and the same year Elfwy, Bishop of York, was consecrated in London, on the festival of St. Juliana. The fleet all chose Knute for king; whereupon advised all the counsellors of England, clergy and laity, that they should send after King Ethelred; saying, that no sovereign was dearer to them than their natural lord, if he would govern them better than he did before. Then sent the king hither his son Edward, with his messengers; who had orders to greet all his people, saying that he would be their faithful lord -- would better each of those things that they disliked -- and that each of the things should be forgiven which had been either done or said against him; provided they all unanimously, without treachery, turned to him. Then was full friendship established, in word and in deed and in compact, on either side. And every Danish king they proclaimed an outlaw for ever from England. Then came King Ethelred home, in Lent, to his own people; and he was gladly received by them all.^2
Just the way this is worded kind of touches the cockles of my blackened and shrivelled heart.
And you know that Æthelred is going to screw it up somehow.
A.D. 1015. This year was the great council at Oxford; where Alderman Edric betrayed Sigferth and Morcar, the eldest thanes belonging to the Seven Towns. He allured them into his bower, where they were shamefully slain. Then the king took all their possessions, and ordered the widow of Sigferth to be secured, and brought within Malmsbury. After a little interval, Edmund Etheling went and seized her, against the king's will, and had her to wife. Then, before the Nativity of St. Mary, went the etheling west-north into the Five Towns, and soon plundered all the property of Sigferth and Morcar; and all the people submitted to him. At the same time came King Knute to Sandwich, and went soon all about Kent into Wessex, until he came to the mouth of the Frome; and then plundered in Dorset, and in Wiltshire, and in Somerset. King Ethelred, meanwhile, lay sick at Corsham; and Alderman Edric collected an army there, and Edmund the etheling in the north. When they came together, the alderman designed to betray Edmund the etheling, but he could not; whereupon they separated without an engagement, and sheered off from their enemies. Alderman Edric then seduced forty ships from the king, and submitted to Knute. The West-Saxons also submitted, and gave hostages, and horsed the army. And he continued there until midwinter.^3
Yep, right on time. Æthelred literally could not go a year without having somebody murdered. His son betrayed him, his hitman attempted to assassinate his son, then defected to the Danes, and Æthelred died shortly thereafter. Literally the worst King of England, King John at least had the support of people who didn't try to murder his heir.
So, Æthelred is dead, his wife (Emma) and children (Aldred and Edward) have been sent into exile in Normandy, where she is from, and it's Cnut versus Edmund:
...wherefore, as both had equal pretensions to the kingdom, since the father of either of them had possessed it, it was consistent with prudence that they should lay aside their animosity, and divide England. This proposition was adopted by either army, and confirmed with much applause, both for its equity and its beneficent regard to the repose of the people who were worn out with continual suffering. In consequence, Edmund, overcome by the general clamour, made peace, and entered into treaty with Canute, retaining West Saxony himself and giving Mercia to the other.^4
Suddenly reasonable people have taken over England. Then, according to Henry of Huntington!
King Edmund was treasonably slain a few days afterwards. Thus it happened: one night, this great and powerful king having occasion to retire to the house for relieving the calls of nature, the son of the ealdorman Edric, by his father's contrivance, concealed himself in the pit, and stabbed the king twice from beneath with a sharp dagger, and, leaving the weapon fixed in his bowels, made his escape. Edric then presented himself to Canute, and saluted him, saying, "Hail! thou who art sole king of England!" Having explained what had taken place, Canute replied, "For this deed I will exalt you, as it merits, higher than all the nobles of England." He then commanded that Edric should be decapitated and his head placed upon a pole on the highest battlement of the tower of London. Thus perished King Edmund Ironside, after a short reign of one year, and he was buried at Glastonbury, near his grandfather Edgar.^5
Now, it goes without saying that you should take the literal account of Henry of Huntington with a grain of salt. That being said, it's not beyond credibility.
It's at this point that I like to make note of the fact that Cnut has two wives (lucky guy): he marries Emma of Normandy, Æthelred's wife, shortly after his victory (I ain't sayin' she's a gold digger...), Emma, of course, leaves the children from the previous marriage behind in Normandy to stay with their uncle. But previously Cnut had married an Ælgifu of Northampton, possibly she was put aside in favour of a better wife. But in the meantime Ælgifu father's name was Elfelm, and her visually impaired brothers are Wulfeah and Ufgeat. Scroll back up to 1006 if you don't get it. Revenge, they say, is a dish best served cold.
So that's how Cnut, a Viking, ended up King of England: he was the least vile person left standing with an army at his back. In the Anglo-Saxon Chronicles the first year of his reign was more or less a list of people he had killed. Most of them probably deserved it. No matter what source you choose Eadric invariably is listed as one of the victims of Cnut's purge.
Sources:
Baxter, Stephen, Simon Keynes, Harold Short, and John Bradley. “Prosopography of Anglo-Saxon England.” Prosopography of Anglo-Saxon England. Accessed October 20, 2014. http://www.pase.ac.uk/index.html.
Henry of Huntingdon. The Chronicle of Henry of Huntingdon. Translated by Thomas Forester. London: Henry Bohn, 1853. https://archive.org/details/chroniclehenryh01foregoog.
Ingram, James, trans. The Anglo-Saxon Chronicles, n.d. http://avalon.law.yale.edu/subject_menus/angsax.asp.
William of Malmesbury. Chronicle of the Kings of England. Translated by John Sharpe and John Giles. London, H. G. Bohn, 1847. http://archive.org/details/williamofmalmesb1847will.