Wisconsin state law says that any person who employs a militia for the protection of property and is not authorized by the state is guilty of a felony. As a minor resident of Illinois, the shooting suspect is not a member of any Wisconsin state militia. His actions were a direct violation of Wisconsin law.

For more info:
Are Militias Legal?

Wisconsin State Legislature: Wisconsin Code of Military Justice

Law & Crime: Lawyers React to Kyle Rittenhouse’s “Well Regulated Militia” Defense

Yahoo!News: Armed militias don’t have legal authority, but today there is little standing in their way