Self defense is an affirmative defense, so the onus is on him to prove that he was acting in self defense, not the other way around.
Anyway, it's a lot more complicated than this. I expect the prosecution to make the argument that he deliberately travelled to the event with an illegal weapon to provoke a situation where he could justify attacking someone in "self defense". That's still murder, even if he was in a genuinely threatening situation. And while Wisconsin has no duty to retreat laws, juries can consider opportunities to retreat when deciding if an act of self defense was actually necessary.
This is also clearly not community policing, since he obviously travelled from Illinois to aggressively confront a community he is not a member of.
Anyway, it's a lot more complicated than this. I expect the prosecution to make the argument that he deliberately travelled to the event with an illegal weapon to provoke a situation where he could justify attacking someone in "self defense". That's still murder, even if he was in a genuinely threatening situation. And while Wisconsin has no duty to retreat laws, juries can consider opportunities to retreat when deciding if an act of self defense was actually necessary.
This is also clearly not community policing, since he obviously travelled from Illinois to aggressively confront a community he is not a member of.