From Wikipedia: Stortorget, scene of the Stockholm Bloodbath, Stockholms blodbad, in November 1520, where, during three days, the Danish king Christian II beheaded and hanged 90 people. On Nov 7th king Christian summoned many Swedish leaders to a talk in Stockholm. But on the 8 November, many of the king's Swedish guests were ambushed and imprisoned. The following day, 9 November, a council, appointed by Christian 2, sentenced the Swedes to death for being heretics; Those opposing the danish kings union of Denmark and Sweden were led out into the great square and beheaded. Surprisingly (to the danish king), the move (by Christian II) didn't go down well with the Swedes, and 300 years of hostilities followed. In the end leading to ruin for the Danish, and victory for the Swedes.
The event earned Christian II the nickname of Kristian Tyrann (Christian Tyrant) in Sweden. It is still a common misconception that danes call him Christian the good, but actually in danish history he is simply known as Christian the second.