To understand the context of the Stockholm Bloodbath, we really need to go back some way, to the workings of the Union of Kalmar, which between 1397 and 1523 joined Sweden (plus Finland) and Norway to the more powerful kingdom of Denmark. The union was a personal one; all four territories were, at least nominally, under the rule of the Danish king. The Swedish nobility largely acquiesced in the arrangement, not least because the sheer difficulty of imposing any sort of direct rule from Copenhagen in effect left them with a largely free hand to run their own affairs as they pleased – and to rule Sweden as constituent members of the Swedish råd (council of state). It was in any case a better arrangement for them than the rule of the German prince Albert of Mecklenberg, who had controlled Sweden until he was was displaced by the imposition of the union, and it better suited the peasantry, who were at least spared the small-scale disasters of endless cross-border raiding.

That said, the union did not always run smoothly. Attempts at tax collection to fund Danish wars and promote Danish interests provoked constant low-level resentment of the arrangement; greater efforts, the announcement of higher taxes, or anything that smacked of an attempt by the Danish king to put himself about Swedish law, could provoke serious crises. In consequence, a Swedish noble, Karl Knutsson, ruled as an effectively independent monarch between 1448 and 1457, again from 1464 to 1465, and for third time between 1467 and 1470 – he is reckoned as King Karl VIII in Swedish king-lists. And from 1471-1520 the country was in the hands of three successive members of the Sture family, who while nominally only regents of the Danish king were to all intents and purposes independent of him. For at least 30 years, the duration of the regency of Sten Sture the Elder, Sweden was again all but an independent country.

Bishops, too, played roles as kingmakers – here we need to be aware of the actions of Jöns Bengtsson Oxenstierna, "the turbulent bishop of Uppsala", and the warlike Kettil Karlsson Vasa of Linköping, both of whom played an important part in the comings and goings of Karl VIII, and both of whom were at one time or another named regent. Finally, the 15th century witnessed the birth of a fourth kind of power in Sweden, beyond that of the Danish king, Swedish nobles and the church: what might loosely be termed an early form of Swedish nationalism, one that idolised the likes of Engelbrekt Engelbrektsson, a gentleman mine-owner who had led popular protests against the Danish king's bailiffs in the 1430s and died as a martyr to this cause; by 1500 Englebrektsson was to all intents and purposes not only the national saint of Sweden, but also a leader who had fought specifically against Danish misgovernment. His memory was kept alive by the Song of Liberty, composed by another bishop, Thomas of Strängnäs, which was probably the only bit of medieval literature well known to every Swede. Thus by the early 1500s, the Danish kings faced a serious challenge to their rule, both in the form of widespread native discontent (armies of Swedish peasants prevailed over Danish mercenaries in 1466 and 1471) and the rise of an effectively hereditary regent family, the Stures, who spent decades building and consolidating their power and who controlled significant and geographically well-focused parcels of land that gave them a good base for exerting economic and military control.

Not everyone who mattered in Sweden was enamoured by the Stures, who by 1510 threatened to become the very thing many Swedish aristocrats feared most – strong, effective, local quasi-kings. The same dislike was felt by some senior members of the Swedish church, since the Stures attempted wherever possibly to dictate the choice of new bishops. This helps to explain the way in which Gustav Trolle became Archbishop of Uppsala: he was nominated by his predecessor, the ancient Jakob Ulvsson, specifically to prevent the Stures from having the chance to nominate a churchman who would bolster their own position. Trolle himself was an old opponent of the regent family; his father had been the main candidate offered as an alternative to the third successive Sture, "Young Herr Sten", to be appointed to the regency. When Trolle was consecrated in Rome, he took the opportunity to secure papal support to use force in defence of the local church's interests and if need be to place the whole of Sweden under interdict – meaning he had the power to suspend almost all of the operations of the church in Sweden.

It is in this context that we need to see the attempts made by the Danish king Kristian II (your Christian the Tyrant) to reassert direct rule over Sweden in 1520, and the concerns of the rest of the Swedish church and nobility, who disliked Danish rule but also distrusted the powerful and authoritarian Stures. There seems to be little doubt that, on his return from Rome, Trolle set out to provoke the Stures and, if possible, secure the dismissal of Young Herr Sten and the return of direct rule from Copenhagen. In 1516, Sture unearthed what he said was a conspiracy to this effect, and 1517 the regent had to repulse a Danish landing. None of this did much to enamour Trolle to his colleagues in the church or to most Swedes. Late in 1517, the Swedish states (effectively the parliament) denounced Trolle as a traitor to his country.

So we have a situation in which Trolle sees himself as a defender of church interests, hopes to secure more power by supporting the restoration of direct Danish rule (with himself, no doubt, as regent), is opposed by a "swearing-in-common" of almost every other Swedish magnate, has the backing of Kristian II (which prevents the rest of the Swedish church from securing his deposition) and is seeking vengeance for his declaration as a traitor. And Trolle soon improved his position, securing additional papal support as a result of the election of Charles V as Holy Roman emperor in 1519 against the wishes of the papacy. Charles, although a brother-in-law of Kristian, was distrusted by the Danish king, and papal desire to build an alliance capable of containing him led the pope to support the laying of the whole of Sweden under interdict. Kristian's invasion of 1520 could thus be presented in the guise of a crusade.

Sten Sture the Younger was mortally wounded in battle against the Danes in January 1520, leaving a son and heir aged only 7 but earning the gratitude of most Swedes for his sacrifice. This was not enough to prevent Kristian from taking Stockholm and forcing the råd to acknowledge him as hereditary king of Sweden. He was swiftly crowned and annointed by Trolle, sweeping away a centuries old elective kingship. All this meant that Trolle – as Kristian's main Swedish supporter – was suddenly placed in a position of great power and influence, but that he was also very widely hated by the majority of the Swedish populace, high and low born.

The archbishop compounded the situation by acting immediately to charge Sture and 17 of his main supporters with heresy. This not only meant the greatest Swedish nobles were threatened with execution, plus confiscation of their estates – it also threatened to place their wealth and property in the power of the Danish king and hence, most likely, of Trolle himself, who was plainly suspected of manoeuvring to have about a million marks worth of their assets made over to him.

This, then, is the background to the Stockholm Bloodbath, which took place on the afternoon on 8 November 1520 and consisted of the beheading of 82 members of the Sture party and the exhumation of Sten Sture the Younger, whose body joined those of the victims of the massacre on three huge funeral pyres. It seems safe to say that no other even in Swedish history (other, perhaps, than the mysterious death of King Karl XII) has been more controversial. The precise sequence of events leading up to the massacre remains hotly disputed, and while it seems generally accepted that Kristian ordered the arrest of the 82, there is no consensus as to how far he approved Trolle's very rapid confirmation of the sentence of death for heresy upon them. Kristian himself laid some of the blame on over-zealous troops, but there are some clues that he was more personally involved, and more approving of the massacre, than he would admit. Several of the most prominent victims were Stockholm burghers who, some historians believe, were disposed of by Kristian so that he could install more pliant successors from outside Sweden, who would help him to rule the country.

Thus it's almost certainly too simplistic to place all the blame at Trolle's feet; he was certainly a major instigator of the trouble, and he was the person who pronounced sentence of death on the victims, but Kristian was still in a relatively vulnerable position, seeking to build a local power base that would make it possible for him to rule effectively in Sweden after his return to Denmark. And one could argue that the actions of the Swedish nobility, church and even peasantry and ordinary townspeople made some sort of demonstration of the king's power necessary if effective Danish rule was to be restored in Stockholm.

Disclaimer: I don't read Swedish and it would be great if this answer could be supplemented by someone who does, and who has access to the most up to date Swedish works on the subject. This response is largely based on the dense analysis offered by Michael Roberts, still the doyen of English-speaking historians of Sweden, in his The Early Vasas (1968).