I’m not a Wisconsin attorney, I’m only licensed in Florida. But in all of the castle defense cases I’ve seen you can’t shoot someone merely on your property, they have to be actively breaking into a building that you are in or actually be in the building.
I’ll give a real life example, one of the minority shareholders in one of my businesses had a black sheep brother who was busted for using drugs. He turned into an informant against his dealer. A few months later he’s sleeping in his parents house when a hired goon busts into the house with a pistol. The brother is woken up and grabs an AK-47 which the goon sees and immediately shoots a couple of times and then flees. Before he makes it to the door the brother shoots a couple of rounds that misses. While the goon is still on the property running away the brother shoots a couple more times and strikes him at least three times before he hits the public sidewalk and drops dead.
In that case the castle defense didn’t apply because he had fled outside the building. If the brother had shot the goon before he hit the door then all would have been good. But because those missed and he shot him in the yard while the goon was allegedly (and probably) fleeing, the castle defense was no longer valid and the brother was sentenced to I believe 15 years in jail on a third degree murder charge.
Similarly, the protestor was outside the building and from what I can tell was fleeing when he was shot. Plus typically castle defenses only applies to the owners and residents of the building and not mere servants/employees.