B) The aristocracy, usually those in the Royal Council. These had no particular objections to the Union (indeed, most were pro) as long as it was a weak union that left them in charge of their individual domains. Generally speaking they wanted a weak union, but a union nonetheless.
A lot were anti union too. There are a lot of writings and poems from this time done by the aristocracy and different rich merchants and such from different parts of Sweden between 1440-1510 about how to slaughter the Danes, throw them out of the nation and create liberty to the Swedes.
Many consider the Kalmar Union the birth of nationalism in Sweden (Alf Henriksson would be one of the more profiled and respected).
One of the main reasons I suspect was the Danish enjoyment of hiring pirates to drive in taxes. Pirates may turn a bit harsh which promote violence.
Here in Norrland there were plenty of peasant risings under the Kalmar Union killing those pirates, burning down the Danish fortress Faxeholmen etc.