Based on
Wisconsin’s self-defense statute, it appears that there are certain situations where a person does not have to retreat to use deadly force - namely if it occurs in the dwelling, motor vehicle or place of business of the person claiming self-defense (typically referred to as the castle doctrine). There is also a provocation element where a person has a duty to retreat before using deadly force in self-defense if that person engaged in unlawful conduct likely to provoke an attack from others and did ultimately provoke the attack in question. I know your position is that he did not provoke the attack, and also that he retreated, but I do suspect these issues (provocation and duty to retreat) will be relevant issues of inquiry in this case.
Edit to add: I’m not a criminal lawyer. Also, I haven’t seen the video footage or delved into all of the relevant facts. That said, based solely on the Wikipedia write-up I just read, the self-defense claim does seem plausible, though going three for three will be a challenge. But I leave that debate to those of you who are more informed. I just wanted to post the relevant statutory language for the sake of the discussion.