England had been ravaged by Danish raids every year from 997 to 1001, and in 1002 the king was told that the Danish men in England "would faithlessly take his life, and then all his councillors, and possess his kingdom afterwards". In response, he "ordered slain all the Danish men who were in England".
Historians believe there was significant loss of life, though evidence is lacking on any specific estimates. Among those thought to have been killed is Gunhilde, who may have been the sister of King Sweyn I of Denmark. Her husband Pallig Tokesen, the Danish Ealdorman of Devonshire, may also have died in the massacre or, according to a different version, played a part in provoking it by his defection to join raiders ravaging the south coast.
For it is fully agreed that to all dwelling in this country it will be well known that, since a decree was sent out by me with the counsel of my leading men and magnates, to the effect that all the Danes who had sprung up in this island, sprouting like cockle amongst the wheat, were to be destroyed by a most just extermination, and thus this decree was to be put into effect even as far as death, those Danes who dwelt in the afore-mentioned town, striving to escape death, entered this sanctuary of Christ, having broken by force the doors and bolts, and resolved to make refuge and defence for themselves therein against the people of the town and the suburbs; but when all the people in pursuit strove, forced by necessity, to drive them out, and could not, they set fire to the planks and burnt, as it seems, this church with its ornaments and its books. Afterwards, with God's aid, it was renewed by me.
The skeletons of 34 to 38 young men, all aged 16 to 25, were found during an excavation at St John's College, Oxford, in 2008. Chemical analysis carried out in 2012 by Oxford University researchers suggests that the remains are Viking; older scars on the bones provide evidence that they were professional warriors. It is thought that they were stabbed repeatedly and then brutally slaughtered. Charring on the bones is consistent with historical records of the church burning (see above).
Historians have generally viewed the massacre as a political act which helped to provoke Sweyn's invasion of 1003. Simon Keynes in his Oxford Online DNB article on Æthelred described it as a "so-called" massacre, the reaction of a people who had been slaughtered and pillaged for a decade, directed not at the inhabitants of the Danelaw but at the mercenaries who had turned on their employers. Æthelred's biographer, Ryan Lavelle, also questions its extent, arguing that it could not have been carried out in the Danelaw, where the Danes would have been too strong, and that it was probably confined to frontier towns such as Oxford, and larger towns with small Danish communities, such as Bristol, Gloucester and London. He comments on the remarkable lack of remorse shown by Æthelred in the Oxford charter, but views the massacre not so much as a royally executed order as an exploitation of popular ethnic hatred and millenarianism. Audrey MacDonald sees it as leading on to the onslaught which eventually led to the accession of Cnut in 1016.