While the Castle Doctrine does provide protection for gun owners involved in shootings occurring in their home, car or business, it does not provide any immunity for the use of a weapon outside of these areas. This leaves a very large void for WI CCW license holders who engage in a self-defense shooting while they are out in public and opens them up to legal fees if charged with a crime or if sued by their target, the target's family, the business owner where the shooting occurs or other bystanders.
Most WI CCW license holders are under the incorrect assumption their home or personal liability umbrella policy will cover them if they are charged with a crime or sued for their actions. This can be a very costly mistake and is an unnecessary risk taken by most CCW license holders that can be avoided with a specific Firearms Liability Policy. A jury trial for a criminal defense can cost $50,000 - $100,000 or more.
Considering the fact that most gun nuts don't think far ahead, much less of others, don't expect them to think of getting insurance. Not that money will help you much when you have a bullet in you.
H/T Phil Scarr