There isn’t a lot of info out there on Kael Merrie, but you can find a bit more on the Roermond Witch Trial. Technically, the trial started in 1613, however, there were quite a few documented cases that took place before that.
In the 1500s many men and women (mostly women) were accused of various crimes and tried as witches. The process was sketchy in the Netherlands at the time. People could accuse you- but unless they had four or five witnesses to back-up the claim, you would only be banished.
Kael was suspected of witchcraft in 1581. They blamed her for a few tragedies in town: a paralyzed pig, a sick child, someone with a cow who’s milk could not be churned… you know, the usual gossip fuel (what?!?!) She denied the charges, but was banished from her home and sent packing. On her way out of town, Spanish mercenaries took justice into their own hands and drown her in the Maas River. This was common practice- even for women whom the charges were dismissed!
During the Trial of 1613, 64 people were burned to death on Galgeberg Hill. This was the largest witch hunt in the Netherlands and it was most likely fueled by religious turmoil and widespread plague.